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B     M     DflS     l.b3  i  INTERIOR 

iU  i3j.AX£;i3  vjrxiiV^juv^GICAL  SURVEY 

GEORGE  OTIS  SM1.TH,  DlBBcroE 

BUKLETIN    588 


THE  CONSTITUTION  OF  THE 
NATURAL  SILICATES 


BY 


FRANK  WIGGLESWORTH  CLARKE 


WASHINGTON 

GOVERN       ENT    PRINTING    OPFIOB 
1914 


MEMCAL    ^Clnl®(DL 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

IVIicrosoft  Corporation 


http://www.archive.org/details/constitutionofnaOOclarrich 


DEPARTMENT  OF  THE  INTERIOR 
UNITED  STATES  GEOLOGICAL  SURVEY 

GEORGE  OTIS  SMITH,  Dibkctoe 


BUtiLETIN   588 


THE  CONSTITUTION  OF  THE 
NATURAL  SILICATES 


BT 


FRANK  WIGGLESWORTH,  CLARKE 


WASHINGTON 

GOVERNMENT    PRINTING    OFFICE 

1914 


CONTENTS. 


Chapter  I.  Introduction 1 5 

Chapter  II.  The  silicic  acids 10 

Chapter  III .  The  silicates  of  aluminum I9 

General  relations ,  19 

The  nephelite  type 21 

The  garnet  type 24 

The  feldspars  and  scapolites 34 

The  zeolites 40 

The  micas  and  chlorites 51 

The  aluminous  borosilicates 65 

Miscellaneous  species 74 

Chapter  IV.  Silicates  of  dyad  bases 87 

Orthosilicates 87 

Metasilicates 94 

Disilicates  and  trisilicates 107 

Chapter  V.  Silicates  of  tetrad  bases,  titanosilicates,  and  columbosilicates 113 

Appendix 124 

Index 125 

3 


frr»  I  I 


THE  CONSTITUTION  OF  THE  NATURAL  SILICATES. 


By  Frank  Wigglesworth  Clarke. 


CHAPTER  I. 
INTRODUCTION. 

In  the  solid  crust  of  the  earth  the  sihcates  are  by  far  the  most 
important  constituents.  They  form  at  least  nine-tenths  of  the  entire 
known  mass  and  comprise  practically  all  the  rocks  except  the  sand- 
stones, quartzites,  and  carbonates,  and  even  these  exceptions  are  com- 
monly derivatives  of  the  silicates,  which  break  up  under  various  condi- 
tions, yielding  new  bodies  of  their  own  class,  together  with  free  sihca 
and  limestone.  From  the  geologist's  point  of  view,  therefore,  the 
sihcates  are  of  fundamental  importance,  and  a  study  of  their  inner 
constitution  may  be  reasonably  expected  to  shed  light  upon  many 
serious  problems.  For  example,  every  primitive  rock  or  eruptive 
mass  contains  an  aggregation  of  silieates,  each  one  of  which  is  capable 
of  undergoing  chemical  change  in  accordance  with  limitations  imposed 
by  the  structure  of  its  molecules.  When  these  changes  take  place 
secondary  compounds,  alteration  products,  are  formed,  and  in  time  the 
rock  becomes  transformed  into  new  substances,  quite  unlike  those 
which  originally  existed.  A  knowledge  of  the  processes  which  thus 
occur  should  be  apphcable  to  the  study  of  the  rocks  and  should  ulti- 
mately render  it  possible  so  to  investigate  a  metamorphosed  mass  as  to 
clearly  indicate  its  origin.  These  processes  are  dependent  on  chemical 
structure,  and  the  study  of  this  with  regard  to  the  sihcates  is  the  pur- 
pose of  the  present  memoir. 

From  the  standpoint  of  the  chemist  the  problem  under  consideration 
is  one  of  great  importance  but  also  of  great  difficulty.  Some  of  the 
difficulty  is  real,  some  only  apparent.  At  first  sight  the  natural  sih- 
cates appear  to  be  compounds  of  great  complexity,  but  this  difficulty 
becomes  much  less  serious  after  careful  examination.  Few  of  the  nat- 
ural sihcates  exist  in  even  an  approxunately  pure  condition;  many 
that  seem  fresh  have  undergone  traces  of  alteration;  isomorphous  mix- 
tures are  exceedingly  common;  and  much  confusion  is  due  to  defective 
analyses.  By  multiphed  observations  these  difficulties  can  be  ehmi- 
nated  from  the  problem,  but  others  yet  remain  to  be  disposed  of.     The 


6  THE   CONSTITUTION   OF   THE   NATUEAL   SILICATES. 

organic  chemist,  to  whom  most  of  our  knowledge  of  chemical  structure 
is  due,  deals  mainly  with  bodies  of  known  molecular  weight,  which  can 
be  measured  by  the  density  of  a  vapor  or  by  cryoscopic  methods.  To 
the  mineral  chemist  such  knowledge  is  not  available,  for  the  com- 
pounds which  interest  him  are  neither  volatile  nor  soluble,  and  their 
molecular  weights  can  only  be  inferred.  The  simplest  empirical  for- 
mula of  a  siHcate  is  not  necessarily  its  true  formula ;  the  latter  may  be 
a  multiple  or  polymer  of  the  former;  and  here  we  find  a  difficulty 
which  is  at  present  almost  insuperable.  Strong  evidence  can  be 
brought  to  bear  upon  this  side  of  the  question,  but  it  is  only  partial 
evidence  and  not  finally  conclusive.  The  case,  however,  is  by  no 
means  hopeless,  for  even  the  partial  solution  of  a  problem  is  better 
than  no  solution  at  all.  An  approximation  is  some  gain,  and  it  is 
possible  so  to  investigate  the  constitution  of  the  sihcates  as  to  bring 
many  relations  to  light,  developing  formulae  which  express  those  rela- 
tions and  indicate  profitable  lines  for  future  research. 

The  problem  is  open  to  attack  along  several  lines,  and  various 
methods  of  investigation  can  be  brought  to  bear  upon  it.  First,  of 
course,  the  empirical  formula  of  each  sihcate  must  be  definitely  ascer- 
tained, which  involves  the  discussion  of  sufficiently  numerous  analyses 
and  the  efimination  of  possible  errors  due  to  impurity,  alteration,  and 
isomorphous  admixtures.  In  this  work  the  microscope  renders  impor- 
tant service  to  the  analyst  and  makes  his  results  much  more  certain. 
By  the  aid  of  the  microscope  many  supposed  mineral  species  have 
been  proved  to  be  mixtures,  and  the  problem  of  the  silicates  has  been 
thereby  simpHfied.  Indeed,  the  final  outcome  of  such  investigation 
generally  indicates,  for  any  given  natural  sihcate,  simplicity  of  compo- 
sition, and  this  is  what  should  be  expected.  These  compounds  are,  as 
a  rule,  exceedingly  stable  salts,  whereas  complex  substances  are  com- 
monly characterized  by  instabifity.  The  mineral  sihcates  are  formed 
in  nature  under  conditions  of  high  temperature  or  are  deposited  from 
solutions  in  which  many  reactions  are  simultaneously  possible,  and 
these  circumstances  are  strongly  opposed  to  any  great  comphcations 
of  structure.  Furthermore,  they  are  few  in  number,  only  a  few  hun- 
dred at  most  being  known;  whereas,  if  complexity  were  the  rule 
among  them,  sHght  variations  in  origin  should  produce  corresponding 
variations  in  character,  and  millions  of  different  minerals  would  be 
generated.  That  few  variations  exist  is  presumptive  evidence  that 
only  few  are  possible,  and  hence  simphcity  of  constitution  is  reason- 
ably to  be  inferred.  In  fact,  we  find  the  same  small  range  of  mineral 
species  occurring  under  the  same  associations  in  thousands  of  widely 
separated  locahties,  a  few  typical  forms  containing  a  few  of  the  com- 
monest metals  being  almost  universally  distributed-  The  longer  the 
evidence  is  considered,  the  stronger  the  argument  in  favor  of  simple 
silicate  structures  becomes. 


INTRODUCTION.  7 

The  empirical  formula  of  a  silicate  having  been  estabUshed,  its 
physical  properties  may  next  be  considered,  and  of  these  the  crystal- 
line form  and  the  specific  gravity  are  the  most  important.  From 
identity  of  form,  or  complete  isomorphism  between  two  species,  we 
infer  similarity  of  chemical  structure,  and  the  inferences  thus  drawn 
are  often  of  the  highest  value.  On  the  other  hand,  dissimilarity  of 
form  and  identity  of  composition  indicate  isomerism,  as  for  example  in 
the  cases  of  andalusite  and  kyanite,  and  here  again  we  obtain  evidence 
which  bears  directly  upon  the  study  of  chemical  constitution.  From 
the  specific  gravity  the  so-called  molecular  volume  of  a  species  may  be 
computed,  and  that  datum  gives  suggestions  as  to  the  relative  con- 
densation of  a  molecule  in  comparison  with  others  of  similar  empirical 
composition.  For  instance,  leucite  and  jadeite  are  empirically  of 
similar  type,  but  the  latter  has  by  far  the  greater  density,  together 
with  superior  hardness.  It  is  therefore  presumably  more  complex 
than  leucite,  and  this  supposition  must  be  taken  into  account  in  con- 
sidering its  ultimate  formula. 

From  what  may  be  called  the  natural  history  of  a  mineral  still 
another  group  of  data  can  be  drawn,  relating  to  its  genesis,  its  con- 
stant associations,  and  its  alterabihty.  In  this  connection  pseudo- 
morphs  become  of  the  utmost  interest  and,  when  properly  studied, 
shed  much  light  upon  otherwise  obscure  problems.  An  alteration 
product  is  the  record  of  a  chemical  change  and  as  such  has  weighty 
significance.  The  decomposition  of  spodumene  into  eucryptite  and 
albite,  the  transformation  of  topaz  into  mica,  and  many  like  occur- 
rences in  nature  are  full  of  meaning  with  reference  to  the  problem 
now  under  consideration.  Just  here,  however,  great  caution  is  nec- 
essary. Mineralogic  literature  is  full  of  faulty  records  regarding 
alterations,  and  many  diagnoses  need  to  be  revised.  Pseudomorphs 
have  been  named  by  guesses,  based  on  their  external  appearance,  and 
often  a  compact  mica  has  been  called  steatite  or  serpentine.  Every 
alteration  product  should  be  identified  with  extreme  care,  both  by 
chemical  and  by  microscopical  methods;  for  without  such  precau- 
tions there  is  serious  danger  of  error.  Each  supposed  fact  should  be 
scrupulously  verified. 

Closely  allied  to  the  study  of  natural  alterations  is  their  artificial 
production  in  the  laboratory.  The  transformation  of  leucite  into  anal- 
cite,  and  of  analcite  back  into  leucite,  is  a  case  in  point,  and  the  admi- 
rable researches  of  Lemberg  furnish  many  other  examples.  Work  of 
this  character  is  much  less  difficult  than  was  formerly  supposed,  and 
its  analogy  to  the  methods  of  organic  chemistry  renders  its  results 
highly  significant.  Atoms  or  groups  of  atoms  may  be  spHt  off  from  a 
molecule  and  replaced  by  others,  and  the  information  so  gained  bears 
du-ectly  on  the  question  of  chemical  structure.  With  evidence  of  this 
sort  relations  appear  which  could  not  otherwise  be  recognized,  and 


8  THE   CONSTITUTION   OF   THE    NATURAL   SILICATES. 

these  relations  may  be  closely  correlated  with  observations  of  natural 
occurrences. 

Evidence  of  the  same  or  similar  character  is  also  furnished  by  the 
thermal  decomposition  of  silicates,  a  line  of  investigation  which  has 
been  successfully  followed  by  several  investigators.  Thus  garnet, 
when  fused,  yields  anorthite  and  an  olivine ;  talc,  on  ignition,  Uberates 
silica;  and  the  prolonged  heating  of  ripidolite  produbes  an  insoluble 
residue  having  the  empirical  composition  of  spinel.  All  such  facts 
have  relevancy  to  the  problem  of  chemical  constitution,  and  their 
number  could  easily  be  enlarged  by  experiment.  As  yet  the  field 
has  been  barely  scratched  on  the  surface;  upon  deeper  cultivation  a 
goodly  crop  may  be  secured. 

The  artificial  S3nithesis  of  mineral  species,  with  the  alUed  study  of 
crystalline  slags  and  furnace  products,  furnishes  still  more  evidence  of 
pertinent  utility.  But  here  again  caution  is  needed  in  the  interpreta- 
tion of  results.  A  compound  may  be  produced  in  various  ways,  and  it 
does  not  follow  that  the  first  method  which  is  successful  in  the  labora- 
tory is  the  method  pursued  by  nature  in  the  depths  of  the  earth.  The 
data  yielded  by  synthesis  are  undoubtedly  helpful  in  the  determination 
of  chemical  constitution,  but  they  furnish  only  a  small  part  of  the 
proof  needed  for  complete  demonstration,  and  their  apphcability  to 
geologic  questions  is  extremely  limited.  For  the  latter  purpose  they 
are  only  suggestive,  not  final. 

Suppose  now  that  the  empirical  formula  of  a  siUcate  has  been  accu- 
rately fixed,  and  that  a  mass  of  data  such  as  I  have  indicated  are  avail- 
able for  combination  with  it.  Suppose  the  physical  properties  to  be 
determined,  the  natural  relations  known,  the  alteration  products 
observed,  its  chemical  reactions  and  the  results  of  fusion  ascertained ; 
what  then?  It  still  remains  to  combine  these  varied  data  into  one 
expression  which  shall  symboHze  them  all,  and  that  expression  will 
be  a  constitutional  formula.  To  develop  this,  the  established  prin- 
ciples of  chemistry  must  be  intelligently  applied,  with  due  regard  to 
recognized  analogies.  The  grouping  of  the  atoms  must  be  in  accord 
with  other  chemical  knowledge;  they  must  represent  known  or 
probable  sihcic  acids;  and  any  scheme  which  fails  to  take  the  latter 
consideration  into  account  is  inadmissible.  Not  merely  composition, 
but  function  also  is  to  be  represented,  and  the  atomic  linking  which 
leaves  that  disregarded  may  be  beautiful  to  see  but  is  scientifically 
worthless.  A  good  formula  indicates  the  convergence  of  knowledge; 
if  it  fulfills  that  purpose  it  is  useful,  even  though  it  may  be  supplanted 
at  some  later  day  by  an  expression  of  still  greater  generality.  Every 
formula  should  be  a  means  toward  this  end,  and  the  question  whether 
it  is  assuredly  final  is  of  minor  import.  Indeed,  there  is  no  formula 
in  chemistry  to-day  of  which  we  can  be  sure  that  the  .last  word  has 
been  spoken. 


INTEODUCTION.  9 

In  the  development  of  constitutional  formulae  for  the  silicates  it 
sometimes  happens  that  alternatives  offer  between  which  it  is  difficult 
to  decide.  Two  or  more  distinct  expressions  may  be  possible,  with  the 
evidence  for  each  so  strong  that  neither  can  be  accepted  or  abandoned. 
In  such  cases  nothing  can  be  done  but  to  state  the  facts  and  await  the 
discovery  of  new  data,  to  which,  however,  the  formulae  themselves  may 
give  clues.  This  sort  of  uncertainty  is  pecuHarly  common  among  the 
hydrous  siUcates,  and  often  rises  from  the  difficulty  of  discriminating 
between  water  of  crystallization,  so  called,  and  constitutional  hy- 
droxyl.  This  difficulty  is  furthermore  enhanced  by  the  common 
occurrence  of  occluded  water  or  water  in  so-caUed  ''solid  solution," 
and  also  by  the  adsorption  of  water  when  a  mineral  is  pulverized  for 
analysis.  The  serious  nature  of  the  latter  compHcation  was  not  rec- 
ognized until  quite  recently. 

In  discriminating  between  rival  formulae  one  rule  is  provisionally 
admissible.  Other  things  being  equal,  a  sjnnmetrical  formula  is  more 
probable  than  one  which  is  unsymmetrical.  Sjrmmetry  in  a  molecule 
conduces  to  stability;  most  of  the  sihcates  are  exceedingly  stable;  and 
hence  symmetry  is  to  be  expected.  This  rule  has  presumptive  value 
only,  as  an  aid  to  judgment,  and  can  not  be  held  rigidly.  It  expresses 
a  probabifity  but  gives  no  proof.  In  a  problem  like  that  of  the  sili- 
cates, however,  even  a  suggestion  of  this  kind  may  render  legitimate 
assistance. 

There  is  an  extensive  literature  relative  to  the  constitution  of  the 
sihcates,  which,  however,  has  been  well  summarized  by  Doelter,^ 
whose  summary  need  not  be  duphcated  here.  When  necessary  suit- 
able reference  will  be  made  to  the  different  authorities. 

I  Handbuch  der  Mineralchemie,  vol.  2,  pp.  61-109, 1912. 


CHAPTER  II. 
THE   SILICIC   ACIDS. 

If  aU  the  silicates  were  salts  of  a  single  silicic  acid  the  problem  of 
their  constitution  would  be  relatively  simple,  but  this  is  not  the  case. 
Many  silicic  acids  are  theoretically  possible,  and  several  of  them  have 
representatives  in  the  mineral  kingdom,  although  the  acids  them- 
selves, as  such,  are  not  certainly  known.  Their  nature  must  be 
inferred  from  their  salts,  and  especially  from  their  esters,  and  this 
side  of  the  problem  is  the  first  to  be  considered. 

As  sihcon  is  quadrivalent,  its  orthoacid  is  necessarily  represented 
by  one  atom  of  the  element  united  with  four  hydroxyl  groups,  thus — 
Si(0H)4,  or,  structurally: 

H 

I 
O 

H— O— Si— O— H 

I 

A 

To  this  acid,  orthosilicic  acid,  the  normal  silicic  esters  and  many 
common  minerals  correspond.  Its  normal  salts,  reduced  to  their 
simplest  expressions,  may  be  typically  represented  as  follows: 


Types. 

Examples. 

R^SiO, 

(C,H,),SiO, 

R",SiO, 

M&SiO, 

R^^^CSiOJe 

Al,(SiO,)3 

R-SiO, 

ZrSiO, 

Any  silicate  in  which  the  oxygen  atoms  outnumber  the  silicon  atoms 
by  more  than  four  to  one,  as,  for  example,  the  compound  Al2Si05,  must 
be  regarded  as  a  basic  salt. 

By  eUmination  of  water  orthosihcic  acid  may  be  conceived  as  yield- 
ing,  first,   metasilicic   acid,   HgSiOg,   and,   secondly,   the   anhydride, 


SiOj,  thus; 


Si(OH),        0=Si=(0H)2        0-=Si=-0 
10 


THE  SILICIC  ACIDS.  H 

Many  salts  that  correspond  to  metasilicic  acid  are  known,  but  no 
esters  have  yet  been  certainly  obtained.  The  esters  first  described  by 
Ebelmen  were  supposed  to  be  metasilicates,  but  aU  recent  investiga- 
tions have  shown  them  to  be  ortho  compounds,  possibly  more  or  less 
impure.  Troost  and  Hautefeuille,  however,  have  described  an  ester 
having  the  formula  (C^HJgSi.O.^,  which  is  a  polymer  of  a  metasiUcate, 
but  its  true  nature  has  not  been  determined.  The  simplest  formulae 
for  typical  metasilicates  are  as  follows  : 

Types.  Examples. 

R^SiOa  Na^SiOa       • 

R^SiOg  MgSiOg 

R^"2(Si03)3  Al,(Si03)3 

K-CSiOs)^  ZrCSiOe)^ 

The  last  two  examples,  AI2  (8103)3  and  Zr (8103)3,  are  salts  not 
actually  known,  but  theoretically  possible. 

By  the  coalescence  of  two  molecules  of  orthosilicic  acid  and  suc- 
cessive elimination,  molecule  by  molecule,  of  water,  a  series  of  disihcic 
acids  may  be  produced,  thus  : 

Si(OH),  8i=(OH)3  0=Si— OH  0=81— OH 

— H,0      6  — H,0  O        — H,0         0 

Si(OH),  8i=(OH)3  8i=(OH)3        0=8i— OH 

The  first  of  these  new  acids,  orthodisiHcic  acid,  Hg8i207,  is  a  sex- 
basic  acid  of  which  several  esters  are  known.  It  is  therefore  well 
established,  and  a  number  of  minerals  appear  to  be  salts  of  it.  The 
second  acid  is  a  polymer  of  metasilicic  acid,  dimetasilicic,  and  its 
formula  is  H48i206.  The  third  compound,  metadisilicic  acid, 
H28i205,  is  represented  by  no  esters,  but  among  its  salts  are  the 
minerals  mordenite,  ptilolite,  milarite  and  petaHte.  By  removing 
the  last  molecule  of  water  the  group  8120^  would  remain,  a  multiple 
of  8i02. 

By  a  similar  process,  that  is,  by  the  elimination  of  water  from  three 
or  four  molecules  of  orthosilicic  acid,  a  series  of  trisilicic  and  quadri- 
silicic  acids  may  be  theoretically  developed.  These  higher  acids 
offer  many  possibihties  for  isomerism,  just  as  we  know  to  be  the  case 
among  the  hydrocarbons.  For  the  present,  however,  only  the  tri- 
silicic series  need  be  considered,  for  above  that  series  the  long  chains 
of  atoms  would  presumably  be  unstable.  At  all  events  the  higher 
series  are  at  present  unnecessary  for  the  interpretation  of  known 
minerals. 


12 


THE   CONSTITUTION   OF    THE   NATURAL   SILICATES. 


The  trisilicic  acids  are  important  and  develop  as  follows : 


Si=(0H)3 

I 
O 


Two  isomers.  Two  isomers. 

0=Si— OH      Si=(0H)3    0=Si— OH      0=Si~OH 

A       i  A  i 


Si=(0H)2  Si=0 

A       A 


Si==(0H)2        Si=0 

A  A 


Si=(OH), 

A 

Si=(0H)3  Si=(0H)3  Si=(0H)3  0=Si— OH  Si=(OH), 

HsSigO.O  HeSigO^  HeSigO^  H^SigO^  H.SigO^ 


Still  another  acid  is  possible  to  complete  the  series,  HgSigO^,  to 
which,  however,  no  known  minerals  correspond.^  The  first  acid 
of  the  series,  orthotrisiUcic  acid,  has  several  representatives  in  the 
mineral  kingdom.  The  second  and  third,  the  trimetasilicic  acids, 
are  polymers  of  metasilicic  acid  and  make,  with  the  similar  acids  of 
the  previous  series,  four  of  the  same  general  formula,  TiHgSiOg.  To 
these  acids  the  four  known  modifications  of  magnesium  metasilicate 
may  perhaps  correspond.  The  fourth  and  fifth  acids  are  most  impor- 
tant, for  they  represent  the  feldspars  and  appear  also  in  some  micas, 
the  scapolites,  and  several  other  species.  Their  isomerism  is  most 
suggestive  and  possibly  accounts  for  such  pairs  of  minerals  as  ortho- 
clase  and  microcline,  or  eudidymite  and  epididymite,  although  the 
latter  case  is  doubtful.  The  simple  name  trisihcic  acid  may  be 
assigned  to  them,  for  in  abundance  their  salts  outrank  all  the  other 
acids  of  the  series. 

Now,  by  including  the  quadrisilicic  acids  for  the  moment,  ignoring 
isomers,  and  tabulating  the  several  compounds,^  some  interesting 
relations  appear: 

Dehydration  derivatives  of  orthosilicic  acids. 


Orthoacids. 

First 
anhydride. 

Second 
anhydride. 

Third 
anhydride. 

Fourth 
anhydride. 

"  Fifth 
anhydride. 

H4Si04 

H^SiOs-..- 

H4Si206---- 
HeSi30,.... 
HsSi^O,^-- 

Si02 

HfiSioO, 

H^Si^Os-..- 
g^SisOs..-. 
HeSi^On-- 

Sio04 

HoSLOin 

H^SiaO^--.. 

H4Si40,o-- 

SisOe 

H2Si409-.-. 

HioSiAa 

siA- 

1  This  acid  is  assumed  by  Tschermak  to  be  the  acid  of  albite. 

2  This  form  of  tabulation  has  also  been  employed  by  Tschermak,  Zeitschr.  physikal.  Chemie,  vol.  53, 
p.  350, 1905. 


THE   SILICIC   ACIDS.  13 

This  table  can  be  extended  indefinitely,  with  the  result  that  in  each 
vertical  column  every  member  below  the  first  differs  from  the  one 
preceding  it  by  the  addition  of  HjSiOg.  Furthermore,  the  first 
anhydride  in  each  series  is  either  HgSiOg  or  a  multiple  thereof.  That 
is,  we  have  a  number  of  homologous  series,  quite  similar  to  those 
with  which  oi^anic  chemistry  has  made  us  familiar.  The  final  anhy- 
dride in  every  series  will  be  a  multiple  of  SiOg,  and  that  fact  seems  to 
shed  some  light  upon  the  possible  differences  between  quartz  glass, 
tridymite  or  cristobaUte,  and  quartz.  The  commonest  associates  of 
quartz  are  the  trisihcate  feldspars,  to  which  quartz  may  be  related  in 
respect  to  its  molecular  magnitude.  Tridymite  and  cristobalite,  with 
lower  specific  gravity,  are  less  condensed  than  quartz  and  may  belong 
in  the  disihcic  series.  The  still  lighter  quartz  glass  is  perhaps  the 
simplest  molecule  of  all,  SiOg.  This  is  hardly  more  than  pure  specula- 
tion, but  the  observed  relations  are  certainly  suggestive.  The  denser 
forms  of  siUca  are  surely  polymers  of  SiOa. 

In  the  foregoing  discussion  the  silicic  acids  have  been  represented 
by  ^' chain"  formulae,  analogous  to  the  formulae  of  the  aliphatic  hydro- 
carbons. But  ''ring"  formulae  of  several  types  are  also  possible,  and 
some  authorities  prefer  them.     For  example,  one  type  is  as  follows: 

(OH),  (OH),  (0H)3 

^i o Si 


Si 

V 


^\ 


(0H)3 

Such  formula  can  be  extended  indefinitely,  but  no  matter  how  many 
siUcon  atoms  are  introduced  into  the  ring  the  saturated  compound 
wiU  be  a  metasiUcic  acid,  nU^SiO,.  The  successive  anhydrides  wiU 
correspond  empirically  but  not  structurally  to  some  of  the  acids  of 
the  previous  scheme,  although  none  can  be  equivalent  to  the  higher 
orthoacids.  This  limitation  makes  the  ring  system  less  general  than 
the  linear  or  chain  system  of  expressions.  Such  acids  as  H,SiO„ 
H^Sifi^,  and  HgSigOio  are  impossible  under  it. 

By  the  coalescence  of  two  or  more  rings,  such  as  is  common  among 
the  aromatic  hydrocarbons,  still  more  complex  acids  are  conceivable, 
thus: 

O  O 

(OH),=Si'^  ^Si-^  ^Si=(OH), 

vv 


14  THE   CONSTITUTION   OP    THE   NATURAL   SILICATES. 

or  H^SigOg,  isomeric  with  the  important  trisilicic  acids  of  the  chain 

series. 

Again, 
^  0  0  0 

(0H)2==Si  Si  Si  Si=(0H)2 

^y  ^y  \y 

0  0  0 

or  HaSi^Oio,  a  polymer  of  the  disihcic  acid  HaSigOg,  and  so  on  indefi- 
nitely. Here  again  the  limitation  holds  that  the  acids  with  a  higher 
oxygen  ratio  than  appears  in  the  formula  H4Si30g  are  excluded  from 
the  scheme.  With  triple  linkings  of  oxygen  only  one  siUcic  acid  is 
immediately  possible,  namely, 

O 

H— O— Si- O— Si— O— H 

V 

or  HgSiaOg,  another  isomer  of  the  disiUcic  acid  in  the  chain  series. 
Two  such  rings,  however,  may  be  linked  together  by  an  oxygen 
atom,  thus: 

O  O 

H  _-0— Si— O— Si— O— Si— O— Si— O— H 

O  O 

or  HaSi^Og,  an  acid  which  corresponds  to  no  known  compounds.  All 
possible  acids  which  appear  in  the  ring  formulae  are  included  in  the 
chain  system,  at  least  so  far  as  their  empirical  formulae  are  considered. 
It  is  evident,  therefore,  from  what  has  been  already  demonstrated, 
that  the  chain  system  is  the  most  complete  and  general.  It  is  not 
necessary,  in  the  present  state  of  knowledge,  to  go  beyond  it,  although 
this  conclusion  should  only  be  held  tentatively.  It  is  possible  that 
some  of  the  simpler  rings  may  help  to  interpret  some  cases  of 
isomerism.^ 

So  far,  then,  there  are  only  a  moderate  number  of  siHcic  acids 
whose  salts  appear  to  need  consideration  in  interpreting  the  natural 
silicates.     They  are: 

Orthosilicic  acid H4Si04 

Metasilicic  acid HgSiOg 

Orthodisilicic  acid HeSigOy 

Dimetasilicic  acid H4Si206 

Metadisilicic  acid HaSigOg 

Orthotrisilicic  acid HgSigOio 

Trimetasilicie  acid HeSigOg  (two  isomers) 

Trisilicic  acid H4Si308  (two  isomers) 

1  Ring  formulae,  like  some  of  those  given  in  the  text,  are  used  by  Vemadsky,  Zeitschr.  Kryst.  Min.,  vol. 
34,  p.  37,  1901. 


THE   SILICIC   ACIDS.  15 

Many  other  acids  are  theoretically  possible,  and  one  of  them, 
HgSi^Oi^,  is  perhaps  represented  by  Troost  and  Hautefeuille's  ester^ 
(C2H5)8Si40i2.  Salts  of  such  acids  may  occur  in  the  mineral  kingdom, 
but  so  far  as  present  evidence  goes  the  probability  of  their  existence 
is  very  small. 

If  the  natural  siHcates  were  simple  normal  salts  of  a  few  siUcic  acids 
the  problem  of  their  constitution  would  not  be  difficult.  But  rela- 
tively few  of  the  known  species  are  of  this  description;  the  greater 
number  are  double  salts,  and  even  triple  replacements  are  not  uncom- 
mon. Furthermore,  there  are  acid  and  basic  salts  to  be  interpreted, 
and  the  latter  class  offers  the  most  serious  difficulties.  A  basic  meta- 
siHcate,  for  example,  may  have  the  same  empirical  composition  as  an 
orthosihcate,  so  that  its  ratios,  studied  apart  from  other  evidence,  tell 
nothing  as  to  the  class  in  which  it  belongs.  For  instance,  the  formula 
AljSiOg,  which  represents  the  composition  of  three  distinct  minerals, 
andalusite,  sillimanite,  and  kyanite,  admits  of  several  different 
structural  expressions.     As  a  basic  metasificate  it  may  be  written 


/SiOa         Al — O — ^Al 

and  as  an  orthosihcate  it  becomes  either 

0=A1— SiO^^Al,    or   SiO^^ 

If  its  molecular  weight  is  a  multiple  of  that  indicated  by  the  formula 
AlaSiOg,  then  the  possibilities  of  isomeric  structure  become  still  more 
complicated.  Its  composition  alone  does  not  give  its  molecular  struc- 
ture, and  other  evidence,  as  shown  in  the  introduction  to  this  memoir, 
must  be  brought  to  bear  before  the  problem  can  be  even  approxi- 
mately solved.  This  evidence  is  sometimes  available,  sometimes  not, 
as  will  be  seen  in  the  systematic  discussion  of  the  individual  species 
later. 

A  similar  but  less  troublesome  difficulty  arises  from  the  common 
occurrence  of  mixed  salts,  which  may  represent  one  or  more  silicic 
acids.  For  example,  a  well-crystallized  silicate  on  analysis  gave 
the  following  empirical  formula:  Na2CaAl4Si8024.  At  first  sight  this 
appears  to  be  a  rather  complicated  metasilicate,  but  microscopic 
evidence  shows  that  the  mineral  is  a  plagioclase  feldspar,  and  the 
formula  then  is  resolvable  into  2NaAlSi308  +  CaAl2(Si04)2,  or,  in  petro- 
graphic  notation,  Ab2Ani .  A  trisihcate  and  an  orthosihcate  have  crys- 
tallized together  in  isomorphous  mixture  and  simulated  a  metasilicate. 


16  THE   COKSTITUTION   OF   THE    N-ATUKAL   SILICATES. 

In  the  interpretation  of  any  silicate,  therefore,  it  becomes  impor- 
tant to  determine  which  acid  it  represents,  and  that  is  not  often  so 
easy  to  do  as  in  the  case  just  cited.  With  some  minerals  the  evidence 
seems  to  be  very  clear,  with  others  it  may  be  misleading.  It  is  only 
by  careful  study  of  a  mineral  in  relation  to  other  species,  and  with 
regard  to  the  alterations  or  chemical  reactions  of  which  it  is  capable, 
that  this  phase  of  the  constitutional  problem  can  be  solved.  A 
noteworthy  attempt  in  this  direction  has  been  made  by  Tschermak,^ 
who  in  a  series  of  researches  has  studied  the  decomposition  of  silicates 
by  hydrochloric  acid  and  sought  to  identify  the  silicic  acids  so  liber- 
ated. Tschermak  has  been  followed  by  Himmelbauer,  Baschieri, 
Silvia  Hillebrand,  and  others.  The  validity  of  his  method  is  sharply 
criticized  by  Miigge.^  Some  of  Tschermak's  conclusions  are  in  har- 
mony with  the  generally  accepted  views ;  but  others  are  at  least  siu*- 
prising.  For  example,  anorthite  and  olivine  are  made  to  be  basic 
metasilicates;  albite  is  a  salt  of  the  acid  HaSigOy;  and  garnet  and  its 
congeners  are  derived  from  trisilicic  acid,  H^SigOg.  Even  if  it  be 
admitted  that  the  acids  obtained  by  Tschermak  are  definite  com- 
pounds, which  has  been  seriously  questioned  by  Miigge  and  Van 
Bemmelen,  what  evidence  is  there  to  show  that  they  represent  in  any 
proper  sense  the  original  minerals?  In  the  decomposition  of  the 
latter  many  reactions  may  occur,  and  the  acids  finally  studied  are 
not  necessarily  those  which  were  first  set  free.  It  is  safe  to  say  that 
the  validity  of  Tschermak's  method  is  not  well  established. 

Siliceous  jellies,  obtained  by  decomposing  alkaline  silicates  with 
acids  or  by  the  hydrolysis  of  SiCl4  or  SiF4,  have  been  studied  by  many 
chemists  with  varying  results.  Norton  and  Roth^  claim  to  have 
prepared  a  definite  orthosilicic  acid  from  SiF4,  but  the  compound  lost 
water  steadily  on  exposure  to  air.  Their  memoir  contains  numerous 
references  to  the  literature  of  the  subject. 

A  hasty  glance  over  the  entire  field  of  the  natural  silicates  will  show, 
first,  that  many  of  them  are  most  easily  interpreted  as  orthosilicates, 
and,  secondly,  that  by  far  the  greater  number  are  salts  of  aluminum. 
As  regards  both  abundance  and  variety  the  aluminous  silicates  out- 
rank all  the  others,  and  from  the  wide  range  of  composition  which 
they  exhibit  we  can  obtain  clues  to  their  constitution.  In  other 
words,  they  furnish  the  most  evidence,  and  some  of  it  is  of  the  highest 
import.  Their  relations  to  one  another  are  oftentimes  clear  and 
unmistakable,  so  that  the  constitution  of  one  salt  is  the  key  to  that  of 
a  second,  and  thus  generalization  becomes  possible. 

1  K.  Akad.  Wiss.  Wien  Sitzungsber.,  vol.  112,  Abth.  1,  p.  355, 1903;  idem,  vol.  115,  Abth.  1,  p.  217, 1906; 
Zeitsehr.  physikal.  Chemie,  vol.  53,  p.  349, 1905;  Centralbl.  Mineralogie,  1908,  p.  225;  Zeitschr,  anorg.  Chemle, 
vol.  63,  p.  230, 1909;  idem,  vol.  66,  p.  199, 1910. 

2  Centralbl.  Mineralogie,  1908,  pp.  129,  325;  and  Van  Bemmelen,  Zeitschr.  anorg.  Chemie,  vol.  59,  p.  225, 
1908.     See  also  Serra,  R.  accad.  Lincei  Atti,  vol.  19,  p.  202, 1910. 

8  Am.  Chem.  Soc.  Jour.,  vol.  19,  p.  832, 1897. 


THE   SILICIC  ACIDS.  17 

The  constitution  of  these  aluminous  silicates  has  been  studied  from 
various  points  of  view.  Thsy  may  be  regarded  as  ordinary  silicates, 
m  which  the  function  of  the  aluminum  is  entirely  basic,  or  as  salts  of 
complex  acids  containing  aluminum  as  part  of  the  negative  radicle. 
That  is,  the  existence  of  alumosilicic  acids  is  assumed  and  the  various 
minerals  are  classed  as  alumosilicates.  This  mode  of  interpretation 
has  been  much  in  vogue  of  recent  years  but  is  open  to  the  objection 
that  it  IS  purely  hypothetical.  Aluminum  may  so  combine  with 
silicic  radicles  as  to  form  complex  ions,  but  that  it  does  so  is  quite 
improved.  Some  writers  have  argued  that  the  aluminum  of  the 
silicates  is  unreplaceable  by  other  basic  radicles,  and  that  the  com- 
pounds in  question  are  thus  different  in  character  from  the  silicates 
of  dyad  bases.  When,  however,  andalusite  or  topaz  alters  to  mus- 
covite,  one-haK  of  the  aluminum  is  replaced  by  the  group  KH2,  and 
so  the  argument  breaks  down.  The  term  "alumosilicate"  may, 
nevertheless,  be  used  as  one  of  convenience,  provided  that  we  remem- 
ber its  limitations.  The  complex  ions  may  exist,  but  they  should 
not  be  taken  too  strenuously  for  granted.  The  fact  that  alumina 
combines  with  silica  is  alone  certain.  That  the  alumosilicates  are 
double  salts,  with  all  of  the  aluminum  basis,  is  just  as  probable  as  the 
alternative  hypothesis.  A  good  summary  of  the  diverse  views  rela- 
tive to  the  alumosilicates  is  given  by  Doelter.*  Some  of  them  will  be 
considered  later  as  regards  their  bearing  on  individual  mineral 
species. 

A  novel  interpretation  of  the  alumosilicates  has  recently  been  put 
forth  by  W.  and  D.  Asch,^  and,  as  it  has  received  considerable  atten- 
tion, it  may  be  briefly  noticed  here.  The  authors  have  developed 
what  they  call  the  "hexite-pentite  theory,"  in  which  rings  of  silicon 
hydrates  are  represented  as  coalescing  with  similar  aluminous  rings. 
These  rings,  as  the  terminology  indicates,  may  contain  either  six  or 
five  atoms  of  silicon  or  aluminum  alternating  with  oxygen  atoms, 
as  in  the  ring  formulae  already  considered  here,  and  they  have  a  super- 
ficial analogy  with  the  benzene  ring  of  organic  chemistry.  The 
silicon  hexite  acid,  HigSigOig,  is  evidently  a  multiple  of  HjSiOg;  the 
aluminum  ring  is  HgAlgOig,  equivalent  to  diaspora.  By  the  coales- 
cence of  four  such  hexite  or  pentite  rings,  either  two  silicic  and  two 
aluminous  or  three  of  one  to  one  of  the  other,  the  authors  develop 
formulae  for  17  alumosilicic  acids,  having  from  6  to  24  replaceable 
hydrogen  atoms,  and  with  molecular  weights  ranging  between  873  and 
1,693.  From  such  complicated  acids  and  their  successive  anhydrides 
the  alumosilicates  are  derived.  The  acids  themselves  with  few 
exceptions,  have  no  representatives  in  nature  and  are  purely  hypo- 

»  Handbuch  der  Mineralchemie,  vol.  2,  pp.  61-109, 1912. 

•  Die  Silicate  in  chemischer  und  technischer  Beziehung,  Berlin,  1911. 

43633°— BuU.  588—14 2 


18  THE   CONSTITUTION  OF   THE   NATURAL  SILICATES. 

thetical.  From  them,  with  so  many  replacements  possible  an  enor- 
mous number  of  salts  can  be  predicated,  and  isomorphous  mixtures, 
altered  or  impure  minerals,  and  even  bad  analyses  may  easily  be 
given  place  in  the  system.  Whether,  however,  silicates  of  corre- 
sponding complexity  could  exist  at  the  temperatures  of  even  the 
coolest  magma  is  most  questionable.  A  generalization  which  does 
too  much  may  be  worse  than  no  generalization  at  all.  Even  for 
such  substances  as  glass  the  authors  of  the  hexite-pentite  theory 
write  structural  formulae. 


CHAPTER  III. 
THE  SILICATES   OF  ALUMINUM. 

GENERAL  RELATIONS. 

A  strictly  logical  investigation  of  the  natural  silicates  might  well 
begin  with  those  of  magmatic  origin,  for  from  them  all  others  have 
been  derived.  Such  a  procedure,  however,  can  not  be  adopted 
exclusively,  for  the  various  compounds,  primary  and  secondary, 
are  connected  by  so  many  interlocking  relations  that  neither  class 
should  be  considered  alone.  This  point  is  well  illustrated  by  the 
primary  alumosilicates,  some  of  whose  derivatives  are  more  sug- 
gestive than  the  original  species.  In  this  class  the  only  simple 
sihcate  which  has  so  far  been  crystalUzed  from  a  molten  mixture  of 
sihca  and  almnina  is  silUmanite,  Al2Si05.  More  complex  salts  are 
easily  generated  from  dry  fusions,  as,  for  example,  nephehte,  leucite, 
and  anorthite,  and  each  one  is  the  progenitor  of  many  secondary 
minerals.  The  problem  of  their  structural  correlation  is  the  problem 
now  in  hand. 

If  we  consider  first  the  orthosihcates  of  aluminum  one  general  rela- 
tion is  easily  made  apparent.  By  a  general  relation  I  do  not  mean  a 
universal  relation,  for  exceptions  are  possible,  but  only  that  many  of 
the  salts  are  connected  by  a  simple  regularity  or  even  a  law.  To  make 
this  clear  it  is  necessary  to  recognize  the  fact  that  aluminum  is 
now  properly  regarded  as  a  trivalent  metal,  its  formerly  supposed 
quadrivalency  being  no  longer  admissible.  Formulae  in  which  alimai- 
num  appears  as  a  tetrad  are  not  vaHd,  and  ferric  iron,  which  replaces 
aluminum,  follows  the  same  rule.  This  point  has  been  established  by 
the  vapor  density  and  cryoscopic  investigations  of  recent  years,  and  is 
sustained  also  by  the  position  of  aluminum  in  the  periodic  classifica- 
tion of  the  elements. 

This  much  admitted,  the  general  relation  of  which  I  have  spoken  is 
as  follows:  Many,  perhaps  all,  of  the  orthosihcates  of  aluminum  are 
most  simply  represented  as  substitution  derivatives  of  the  normal  salt 
AI4  (8104)3.  To  illustrate  this  rule  for  present  purposes,  the  following 
examples  will  suffice : 


Aluminum  orthosilicate  .  .Al4(Si04)3 

Eucryptite Al3(Si04)3Li3 

Nephelite Al3(Si04)3Na3 

Kaliophilite Al3(Si04)3K3 

Muscovite Al3(Si04)3HK3 


Paragonite Al3(Si04)3NaH2 

Kryptotile Al3(Si04)3H3 

Andalusite Al3(Si04)3(A10)8 

Topaz Al3(Si04)3(AlF2)8 

19 


20  THE   CONSTITUTIOlir   OF   THE   NATURAL  SILICATES. 

These  formulae  express  not  only  the  composition  of  the  minerals  but 
also  many  facts  concerning  their  relations,  such  as  their  association, 
their  alteration  one  into  another,  and  so  on.  Thus,  topaz  and  anda- 
lusite  are  crystallographically  akin ;  both  minerals,  as  well  as  others  in 
the  series,  alter  easily  into  muscovite,  and  these  facts  become  intelhgible 
in  the  light  of  the  formulae  given.  In  the  use  of  the  formulae,  however, 
one  possible  misconception  must  be  avoided.  They  express  a  rela- 
tionship of  constitution  but  do  not  imply  that  nature  first  generated 
the  normal  salt  and  then  actually  developed  the  other  compounds  from 
it.  To  emphasize  this  point  an  analogy  may  be  drawn  from  organic 
chemistry.  Alizarin,  derived  constitutionally  from  anthracene,  was 
originally  obtained  from  a  glucoside  contained  in  madder  root.  But 
nobody  supposes  that  the  madder  plant  took  anthracene  as  a  starting 
point  from  which  to  produce  the  dye.  The  constitutional  or  struc- 
tural derivation  is  one  thing;  the  natural  origin  is  quite  another. 

Whether  aluminum  orthosilicate  as  such  occurs  in  nature  is  still 
a  matter  of  doubt.  At  all  events  its  existence  has  not  been  definitely 
established.  It  is  theoretically  possible,  and  an  artificial  hydrate  of 
the  formula  Al4(Si04)3.6H20  has  been  described  by  Pukall.^  As 
regards  its  ultimate  constitution  or  chemical  structure  there  is  much 
uncertainty.  Its  formula  can  be  written  structurally  in  several 
ways ;  as,  for  instance,  with  each  aluminum  atom  linked  with  all  three 
Si04  groups,  or  with  only  one  atom  so  connected.  In  a  sense  this 
problem  is  analogous  to  that  offered  by  the  benzene  ring,  prism,  or 
nucleus,  a  conception  of  which  the  utiHty  is  fully  recognized,  in  spite 
of  outstanding  uncertainties.  For  practical  purposes,  that  is,  for  the 
coordination  of  known  facts,  expressions  like  the  following  are 
sufficient: 

.Si04=Al  ^i04=KH2  ^i04=CaH 

Al— Si04=Al         Al— Si04=Al  Al— SiO^^CaH 

\si04^Al  \si04=Al  \si04=Al 

These  expressions  indicate  the  observed  replaceability  of  aluminum 
atoms  by  other  atoms  or  groups  and  have  no  ulterior  significance.  So 
long  as  their  limitations  are  kept  in  mind  they  are  useful,  but  beyond 
this  it  would  be  unreasonable  to  go.  With  prolonged  discussion  and 
more  evidence  we  may  get  a  deeper  insight  into  the  nature  of  the 
fundamental  molecule;  at  present,  speculation  concerning  it  would  be 
premature.  The  relations  expressed  are  clear,  no  matter  what  others 
may  be  revealed  in  the  future.  As  a  working  hypothesis,  the  concep- 
tion of  substitution  from  a  normal  salt  may  be  a^pHed  to  many  non- 
aluminous  silicates,  as  in  the  magnesian  series,  the  silicates  of  quadriv- 
alent metals,  and  so  on.     These  points  wiU  be  developed  in  subsequent 

1  Deutsche  chem.  Gesell.  Ber.,  vol.  43,  p.  2098, 1910. 


THE   SILICATES   OF  ALUMINUM.  21 

chapters.     For  the  present  we  need  only  to  consider  the  alumosih- 
cates,  group  by  group. 

THE  NEPHELITE  TYPE. 

If,  now,  we  start  out  from  the  normal  aluminum  orthosilicate  the 
first  and  simplest  replacement  possible  is  that  of  a  single  aluminum 
atom  by  three  monads,  giving  a  compound  of  the  general  formula 
AlgCSiOJgR'g.  This  formula  represents  several  well-known  minerals, 
and  I  propose  to  designate  it  the  nephehte  type.  At  first  sight  it 
seems  to  be  reducible  to  the  simpler  expression  R'AlSi04,  but  that 
expression,  as  will  be  seen  later,  does  not  indicate  all  the  known  rela- 
tions of  the  group. 

The  first  three  representatives  of  this  type  are  as  follows: 

Eucryptite AlgCSiOOgLig 

Nephelite ..Al3(Si04)3Na3 

Kaliophilite Al3(Si04)3K3 

These  species  are  all  hexagonal,  are  nearly  equal  in  density,  and  all 
gelatinize  with  hydrochloric  acid.  The  second  and  typical  member 
of  the  series  has  been  made  synthetically,  and  is  then  found  to  have 
the  composition  indicated  hj  the  formula.  The  natural  nephehte, 
however,  has  a  composition  which  is  more  exactly  represented  by  the 
complex  formula  R'3Al3Si9034,  in  which  a  little  potassium  appears 
among  the  components  of  E,',  and  the  sihca  is  in  excess  of  the  amount 
required  by  theory.  The  potassium  is  doubtless  due  to  an  isomor- 
phous  admixture  of  kahophilite,  and  the  excess  of  sihca  can  be  explained 
by  the  presence  of  a  salt  isomeric  with  albite  and  having  the  composi- 
tion Al3(Si308)3Na3.  This  replacement  of  Si04  by  SigOg  appears  to  be 
common  among  the  sihcates,  and  its  recognition  clears  up  many 
discrepancies.  In  this  case  one  molecule  of  the  trisilicate  commingled 
with  fifteen  of  the  ortho  salt  will  produce  the  divergence  from  normal 
composition  shown  in  the  analyses  of  natural  nephehte. 

This  view  of  the  constitution  of  nephehte  has  been  adopted  by 
Schaller,^  and  is  also  favored  by  Bowen,^  who  has  studied  the  fusion 
diagram  of  the  system  silica,  alumina,  and  soda.  In  that  investiga- 
tion Bowen  found  that  at  about  1,550°  nephehte  is  transformed  into 
a  trichnic  isomer,  a  soda  anorthite,  to  which  the  name  carnegieite  has 
been  given.  Foote  and  Bradley,^  however,  have  advanced  a  shghtly 
different  interpretation  of  the  anomalous  composition  of  natural 
nephelite,  ascribing  the  excessive  sihca  to  ''solid  solution. '^  These 
three  researches  represent  the  most  recent  and  most  conclusive  work. 
Morozewicz  *  has  explained  the  divergent  analyses  of  nephehte  by 
assuming  a  series  of  different  nephehte  molecules,  derived  from  a 

1  Washington  Acad.  Sci.  Jour.,  Sept.  19, 1911.  »  Am.  Jour.  Sci.,  4th  ser.,  vol.  33,  p.  439, 1912. 

2  Am.  Jour.  Sci.,  4th  ser.,  vol.  33,  pp.  49,  551, 1912.       *  Acad.  Cracovie  Bull.,  1907,  p.  958. 


22  THE  CONSTITUTION   OF   THE   NATURAL  SILICATES. 

number  of  alumosilicic  acids.  That  interpretation  seems  to  be  no 
longer  tenable.  A  normal  mineral  (Na;K)AlSi04  has  been  described 
by  Zambonini  ^  under  the  name  of  pseud  on  ephelite.  It  is  evidently 
a  mixture  of  nephelite  and  kaliophilite.  The  equivalency  of  these 
species  is  also  clearly  proved  by  an  experiment  of  Lemberg,^  who 
heated  nephelite  (elseolite)  with  a  solution  of  potassium  silicate  and 
obtained  a  product  having  the  composition  of  kahophihte. 

Eucryptite  and  nephelite  both  alter  with  great  ease  into  muscovite, 
a  potassium  salt  of  which  paragonite  is  the  sodium  equivalent.  Fur- 
thermore, C.  and  G.  Friedel,^  by  heating  finely  divided  muscovite  to 
500°  in  a  solution  of  alkali,  obtained  nephehte  in  crystals.  From  this 
evidence  the  formulae  of  muscovite  and  paragonite  become  directly 
related  to  those  of  the  nephelite  series,  thus : 

Nephelite Al3(Si04)3Na3 

Muscovite Al3(Si04)3KH2 

Paragonite Al3(Si04)3NaH2 

Physically,  the  two  micas  have  no  resemblance  to  nephehte,  being 
different  in  form,  sHghtly  denser,  and  refractory  toward  acids.  The 
relationship  is  purely  one  of  chemical  type,  and  is  estabHshed  by  the 
fact  of  alteration  from  one  into  another. 

Kryptotile,  according  to  UliHg,*  is  an  end  member  of  the  mica  group. 
If  so,  its  formula  becomes  Al3(Si04)3H3.  The  clayhke  mineral 
leverrierite  has  apparently  the  same  composition  and  may  be  the 
same  compound,  and  another  clay,  rectorite,  is  similar  but  with  one 
additional  molecule  of  water,  which  is  lost  at  110°.  The  compound 
may  be  regarded  as  an  alumosihcic  acid,  with  three  replaceable  hydro- 
gen atoms,  although  such  an  interpretation  of  it  is  not  necessary. 

Through  muscovite  a  connection  is  recognizable  between  the  forego- 
ing species  and  the  two  minerals  andalusite  and  topaz,  whose  simplest 
formulae,  tripled,  may  be  written  as  follows : 

Topaz Al3(Si04)3(AlF2)3 

Andalusite Al3(Si04)3(A10)3 

Here  we  encounter  the  evidently  univalent  atomic  groups 

F 
— A1=0  and  — Al/ 

\f 

both  of  wliich  play  an  important  part  in  various  other  minerals.  The 
two  species,  topaz  and  andalusite,  are  closely  alHed  crystallograph- 
ically.  They  have  sensibly  identical  molecular  volumes,  and  both 
undergo  alteration  into  muscovite  mica.^     In  topaz,  as  shown  by  the 

1  Chem.  Soc.  Jour.,  vol.  98,  pt.  2,  p.  1078, 1910.    Abst.  from  Accad.  Napoli  Rend.,  1910. 

2  Deutsche  geol.  Gesell.  Zeitschr.,  vol.  37,  p.  966,  1885. 
8  Min.  Soc.  Bull.,  vol.  13,  p.  183, 1890. 

*  Zeitschr.  Kryst.  Min.,  vol.  47,  p.  215, 1910. 

6  For  a  good  example  of  this  alteration  see  Clarke  and  Diller,  U.  S.  Geol.  Survey  Bull.  27,  p.  9, 1886. 


THE   SILICATES   OF  ALUMINUM.  23 

investigations  of  Penfield  and  of  Jannasch,  hydroxyl  commonly 
replaces  a  part  of  the  fluorine,  hydroxyl  and  fluorine  being  clearly 
isomorphous.  The  formula  given  is  that  of  normal  topaz,  entirely 
free  from  alteration. 

Obviously  the  formula  of  muscovite  is  the  key  to  all  other  formulae  in 
this  group  of  siHcates.  Its  minimum  molecular  weight  is  represented 
by  the  expression  AlgKHaSigOia  and  to  that  the  others  must  conform. 
The  general  formula  Al3(Si04)3R'3  is  the  lowest  possible,  and  the 
formula  NaaAIjSiaOg,  which  is  often  assigned  to  nephelite,  is  too  small. 
It  may  represent  the  isomeric  carnegieite,  which  being  stable  at  high 
temperatures  is  perhaps  molecularly  less  condensed  than  nephelite. 
But  of  this  there  is  no  clear  evidence.  The  tripled  formulae  are  also 
sustained  by  an  experiment  of  Silber,^  who  heated  an  artificial  nephe- 
lite silicate  in  a  sealed  tube  with  a  solution  of  silver  nitrate  and  replaced 
one- third  of  the  sodium  by  silver:  that  is,  one  of  the  three  sodium 
atoms  seems  to  be  differently  combined  from  -the  others.  This  sub- 
stitution can  be  expressed  structurally  in  several  ways,  but  its  con- 
sideration must  be  deferred  until  later.  Nephelite  yields  some 
zeoHtic  derivatives,  especially  hydronephelite  and  natroUte,  but  their 
discussion  belongs  to  another  section  of  this  chapter. 

To  sum  up,  we  have  now  eight  definite  species  represented  by  the 
fundamental  type  Al3(Si04)3ll'3,  the  first  substitution  from  the 
hypothetical  normal  orthosilicate  of  aluminum,  and  these  compounds 
may  be  divided  into  three  subtypes  as  follows: 

Nephelite.  Muscovite.  Topaz. 

^iO^^Naa  ^iO,=KH2  .SiO,={AlF,\ 

Al— SiO,=Al  Al— SiO,=Al  Al— SiO,=Al 

\siO,=Al  \siO,=Al  \siO,^Al 

symbols  which  clearly  indicate  the  known  chemical  relations  between 
the  several  minerals.  In  six  of  the  eight  examples  the  simplest  pos- 
sible formulae  have  been  tripled,  for  otherwise  the  relationships  which 
exist  could  not  be  structurally  shown.  The  correctness  of  this  pro- 
cedure will  appear  stiU  more  definitely  in  the  consideration  of  the 
groups  which  foUow. 

The  species  silUmanite  is  isomeric  with  andalusite,  but  the  structural 
character  of  the  isomerism  is  not  clear.  The  two  species  have  nearly 
the  same  molecular  volumes,  and  presumably  the  same  molecular 
weights,  but  a  third  isomer,  kyanite,  is  much  denser  and  therefore  not 
so  easily  correlated  with  the  others.  It  is  commonly  regarded  as  a 
basic  metasiUcate,  although  that  is  not  its  only  conceivable  structure. 
It  is  easy  to  write  constitutional  formulae  for  all  these  minerals,  but 
they  would  be  of  httle  real  significance  except  in  so  far  as  they  repre- 

1  Deutsche  chem.  GeseU.  Ber.,  vol.  14,  p.  941, 1881. 


24  THE   CONSTITUTION    OF   THE    NATURAL   SILICATES. 

setited  possibilities.  Sillimanite  is  the  most  stable  compound  of  the 
three,  and  the  only  one  which  has  been  obtained  magmatically.  At 
high  temperatures  kyanite  and  andalusite  are  transformed  into 
sillimanite.^  Structural  formulae  for  andalusite  and  kyanite  have 
been  proposed  by  Zulkowski,^  but  they  are  based  upon  the  minimLum 
molecular  weight  of  AlgSiOg  and  are  therefore  inadmissible. 

THE  GARNET  TYPE. 

By  this  title  I  propose  to  designate  the  second  series  of  derivatives 
from  the  normal  salt,  Al4(Si04)3,  in  which  two  atoms  of  aluminum 
have  been  replaced.  The  general  formula  of  the  type  obviously  is 
Al2(Si04)3ll'6;  and  in  this  series  bivalent  elements  or  radicles  fre- 
quently appear.  In  lagoriohte,  an  artificial  soda  garnet,^  Il'6  =  Na6; 
in  prehnite  B.\  =  00,2^.2}  ^^^  i^i  normal  garnet  and  epidote  R'g  =  3R''. 
There  are,  therefore,  three  subtypes  to  consider — one  in  which  all  the 
replacing  atoms  are  univalent,  one  in  which  all  are  bivalent,  and  one 
intermediate  between  the  other  two. 

Under  the  first  subtype  two  species  may  be  definitely  placed, 
namely,  lagoriolite,  Al2(Si04)3Na6,  and  zunyite,  which  is  more  com- 
plicated. In  zunyite  R'g  is  composed  of  the  univalent  radicles 
— ^A1=F2,  — Al=Cl2,  and  — A1==(0H)2,  but  the  species  has  been 
found  in  only  one  locaHty,  and  needs  further  study.  At  present, 
if  we  unite  the  chlorine  in  it  with  the  fluorine,  it  may  be  provisionally 
represented  by  the  expression 

.SiO,=(A102H2)2.AlF2 
Al— Si04=(A102H2)2.AlF2 
\si04=Al   . 

This  formula  expresses  the  facts  which  are  now  available  but  is  not 
conclusive.  Its  isometric  character,  however,  helps  to  connect 
zunyite  with  the  garnet  and  sodalite  groups,  as  has  been  shown  by 
Brogger.'^ 

In  the  second  subtype,  when  R'q  is  partly  composed  of  bivalent  and 
partly  of  univalent  atoms,  two  species  may  be  placed,  thus: 

Prehnite Al2(Si04)3Ca2H2 

Biotite Al2(Si04)3Mg2HK 

Possibly  the  tetragonal  sarcolite,  which  has  the  general  formula  of  a 
garnet  with  the  lime  partly  replaced  by  soda,  may  fall  here  also,  but 
the  analyses  of  this  mineral  are  unsatisfactory,  and  its  relations  are 

1  Vemadsky,  Sex;.  Min.  Bull.,  vol.  12,  p.  447, 1889;  vol.  13,  p.  256, 1890. 

2  Monatsh.  Chemie,  vol.  21,  p.  1086,  1900. 

3  See  Morozewlcz,  Min.  pet.  Mitt.,  vol.  18,  p.  147, 1898-99.  The  formula  here  given  to  lagoriolite  is  that  of 
the  ideally  pure  mineral.  The  actual  product  contains  a  notable  admixture  of  the  corresponding  lime 
compound. 

*  Zeitschr.  Kryst.  Min.,  vol.  18,  p.  209, 1891. 


THE   SILICATES  OF  ALUMINUM.  25 

still  uncertain.     Biotite  will  be  more  fully  considered  in  the  section 
devoted  to  the  mica  group. 

Under  the  third  subtype  of  this  series  we  find  the  garnet  group 
itself,  together  with  epidote  and  several  related  species.  The  sodalite 
group  is  also  akin  to  garnet  and  to  the  second  subtype  and  will  be 
considered  in  this  connection  a  little  later.  The  generic  term  garnet 
covers  several  species,  all  isometric  and  strictly  isomorphous,  in  which 
magnesium,  calcium,  and  ferrous  iron  replace  one  another,  and 
chromium,  aluminum,  and  ferric  iron  are  also  equivalent  terms.  Thus 
we  have: 

Grossularite Al2(Si04)3Ca3 

Pyrope Al2(Si04)3Mg3 

Almandite Al2(Si04)3Fe^^3 

Spessartite Al2(Si04)3Mn3 

Andradite Fe2(Si04)3Ca3 

Ouvarovite Cr2(Si04)3Ca3 

To  these  may  be  added  schorlomite,  a  garnet  in  which  titanium 
occurs  both  as  part  of  the  acid,  that  is,  with  Ti04  replacing  Si04,  and 
also  as  Ti'^'  among  the  triad  bases,  equivalent  to  aluminum.  The 
monoclinic  partschinite,  isomeric  with  spessartite,  also  falls  into  this 
group. 

The  several  species  of  garnet  occur  in  a  great  variety  of  isomorphous 
mixtures  and  some  of  them  contain  small  quantities  of  alkalies,  due  to 
the  presence  of  compounds  like  lagoriohte.^ 

In  the  epidote  group  several  species  appear,  one,  zoisite,  being 
orthorhombic,  whereas  the  others  are  monoclinic.  These  species  are 
characterized  by  the  bivalent  group  of  atoms  =A1 — OH  or  =Fe — OH, 
thus: 

Zoisite Al2(Si04)3Ca2(A10H) 

Epidote  a rAl2(Si04)3Ca2(A10H) 

Epidote  6 lFe2(Si04)3Ca2(FeOH) 

Piedmontite (Al,Mn)2(Si04)3Ca2(  AlOH) 

Allanite (Al,Ce,Fe)2(Si04)2(Ca,Fe)2(A10H) 

Hancockite Al2(Si04)3(Ca,Pb,Sr)2(FeOH) 

or,  in  general,  as  compared  with  garnet. 

Garnet R^^^2(Si04)3ll^^3 

Epidote W,{SiO,),WyW^'OB) 

A  chromium  epidote,  ''tawmawite,"  containing  11.16  per  cent  of 
Crfia  has  also  been  described. ^ 

The  facts  that  garnet  alters  into  epidote  and  that  the  two  minerals 
are  often  associated  give  emphasis  to  the  formulae. 

1  For  elaborate  studies  of  the  garnet  group  see  Brogger  and  Backstrom,  Zeitschr.  Kryst.  Min.,  vol.  18, 
p.  209,  1891;  Weinschenk,  idem,  vol.  25,  p.  365,  1S96;  Uhlig,  Naturh.  Ver.  preuss.  Rheinl.  u.  Westfalens, 
Verb.,  vol.  67,  pt.  2,  p.  307,  1910;  and  Seebach,  Centralbl.  Mineralogie,  p.  774, 1906. 

2  See  Bleeck,  India  Geol.  Survey  Records,  vol.  36,  p.  254, 1907-8. 


26  THE   CONSTITUTION   OF    THE   NATURAL   SILICATES. 

Although  garnet  as  a  rule  is  unattacked  by  acids,  and  epidote  is 
only  in  part  decomposable,  both  species  are  so  broken  up  by  strong 
ignition  as  to  be  readily  acted  upon  by  hydrochloric  acid,  with  separa- 
tion of  gelatinous  silica.  According  to  Doelter  and  Hussak,^  garnet 
yields  upon  fusion  sometimes  anorthite  and  an  olivine;  or  meionite, 
augite,  and  olivine;  or  melilite  and  anorthite;  and  occasionally  spinel. 
Epidote,  says  Doelter,^  yields  lime-augite  and  anorthite,  and  prehnite 
behaves  like  garnet.  These  facts  are  interesting,  but  they  give  no 
direct  information  regarding  chemical  structure.  By  fusion  the 
molecules  of  a  silicate  are  broken  down,  and  on  cooling  the.  melt  a 
complete  rearrangement  of  the  atoms  may  take  place,  although  not 
necessarily.  When  calcium  alumosilicates  are  fused  they  may,  as 
in  the  case  of  anorthite,  recrystallize  unchanged,  or  they  may  solidify 
as  compounds  having  little  or  no  structural  relations  to  their  pro- 
genitor. When,  however,  silicates  are  broken  down  by  mere  calcina- 
tion and  without  fusion  the  reaction  may  be  highly  instructive. 
Examples  of  this  kind  will  be  noted  later. 

In  the  four  species  sodalite,  haiiynite,  noselite,  and  lazurite  we 
have  a  group  of  minerals  which  Brogger  has  classified  as  alkali  gar- 
nets.^ Like  garnet,  they  are  all  isometric,  and  they  are  characterized 
by  the  presence  of  the  bivalent  groups  =A1 — CI,  =A1 — SO4 — Na,  and 
=A1 — S — S — S — Na.  There  are  also  artificial  products,  ultra- 
marines, in  which  the  groups  =A1 — S — S — Na  and  ^Al — S — Na 
appear.  By  adopting  Brogger's  formulae,  which  are  preferable  to 
those  formerly  proposed  by  myself,^  these  species  may  be  written  as 
follows : 

Sodalite Al2(Si04)3Na4(AlCl) 

Haiiynite •- Al2(Si04)3Na2Ca(AlS04Na) 

Noselite Al2(Si04)3Na4(AlS04Na) 

Lazurite Al2(Si04)3Na4(AlS3Na) 

They  fall  therefore  properly  under  the  second  subtype,  but  are  con- 
sidered at  this  point  on  account  of  their  analogies  with  garnet. 

The  formulae  just  assigned  to  these  minerals  represent,  of  course, 
the  ideally  pure  compounds,  which  rarely,  if  ever,  occur  in  nature. 
The  four  species  are  all  evidently  derived  from  nephelite,  with  which 
sodalite  is  commonly  associated,  and  their  composition  varies  in  the 
same  manner  as  that  of  the  parent  mineral.  Like  nephelite,  sodalite 
yields  natrolite,  hydronephelite,  and  muscovite  by  alteration. 
Furthermore,  C.  and  G.  Friedel,^  on  heating  powdered  muscovite  with 
soda  solution  and  sodium  chloride  at  a  temperature  of  500°,  obtained 

1  Allg.  chem.  Mineralogie,  p.  182, 1890. 

2  Idem,  p.  183. 

»  Brogger  and  Backstrom,  Zeitschr.  Kryst.  Min.,  vol.  18,  p.  209, 1891. 
*  U.  S.  Geol.  Survey  Bull.  42,  p.  38,  1887. 
6  Soc.  min.  Bull.,  vol.  13,  p.  183, 1890. 


THE   SILICATES   OF  ALUMINUM.  27 

sodalite  artificially,  although  nephelite  was  probably  first  formed  as 
an  intermediary,  and  the  two  species  were  commingled  in  the  product. 

The  two  hexagonal  species,  cancrinite  and  microsommite,  are  also, 
like  sodalite,  undoubtedly  derivatives  of  nephelite,  but  their  formulae 
are  rather  uncertain.  At  Litchfield,  Maine,  cancrinite  often  occurs 
in  intimate  mixture  with  nephelite  (elseolite).  A  cancrinite 
described  by  Zambonini  ^  corresponds  very  closely  to  a  mixture  of 
nephelite  and  the  compound  Al2(Si04)3Na3Ca(AlC03). 

In  its  purest  varieties  cancrinite  approximates  to  the  formula 
Al2(Si04)3Na4H(AlC03),  in  which  a  little  soda  is  replaced  by  lime,  and 
the  univalent  group  — A1=C03  may  be  partly  substituted  by  — Al= 
Si03.  Mcrosommite,  according  to  the  published  analyses,  varies 
widely  in  composition,  invariably  containing  potassium  and  having  a 
notable  proportion  of  chlorine  and  SO3  among  its  constituents. 
If,  however,  we  assume  in  it  the  univalent  radicles  — Al=Cl2  and 
— A1=S04,  its  composition  reduces  easily  to  the  form  Al2(Si04)3 
(NaK)3Ca(Al(S04Cl2)),  like  cancrinite,  both  species  having  then  the 
composition  of  the  general  type  Al2(Si04)3R'e.  The  theory  as  pro- 
posed, then,  assumes  univalent  complex  radicles  for  cancrinite  and 
microsommite,  and  bivalent  radicles  for  the  sodalite  group,  thus : 

In  cancrinite  group.  In  sodalite  group. 

— Al=-Cl2  =A1— CI 

— A1=S04  =A1— SO4— Na 

— A1=C03  =A1— S3— Na 
— Al=Si03 

and  the  typical  structures  are  as  follows: 

Cancrinite.  Sodalite. 

.Si04=Na2(AlC03)  /Si04^^1__Cl 

Al^Si04=Na2H  At— Si04=Na2 

\si04=Al  \si04=Al 

The  best  analyses  of  microsommite  give  very  nearly 

^i04=Na2-AlS04  ^i04=Na2.AlCl2 

1  Al"Si04=NaCa  +         2  Al— Si04=NaCa 

\si04=Al  \si04=Al 

with  nearly  half  the  sodium  replaced  by  potassium;  the  radicle 
AICO3  is  also  sometimes  present. 

There  are  arguments  both  for  and  against  these  formulae  and  the 
pecuhar  univalent  and  bivalent  radicles  assumed  in  them.  The 
assumption   of   a   group   -Al=Si03,   equivalent   to   and  replacmg 

1  Appendice  alia  mineralogia  vesuviana,  p.  35. 


28  THE   CONSTITUTION   OF   THE   NATURAL  SILICATES. 

— ^A1=C03,  is  clearly  suggested  by  the  experiments  of  Lemberg/  who 
by  the  action  of  sodium  silicate  solution  upon  elseoHte,  obtained  a 
compound  which  he  designates  as  a  cancrinite  containing  Na2Si03  in 
place  of  NagCOg.  By  similar  reactions  with  sodium  carbonate  he  pro- 
duced a  substance  having  the  composition  of  true  cancrinite.  Hence, 
whatever  the  ultimate  molecular  structure  of  cancrinite  may  be,  we 
are  amply  justified  in  assuming  in  it  the  replaceability  of  CO3  by  SiOg. 

These  experiments  fairly  represent  a  large  number  of  like  kind  which 
are  due  to  Lemberg,  and  which  are  recorded  in  his  papers.  Some  of 
these  will  be  cited  later,  but  a  reference  to  the  work  of  his  colaborer,^ 
Thugutt,  is  in  place  at  this  point.  Starting  from  a  hydrated  nepheUte, 
artificially  prepared  from  kaolin,  Thugutt  succeeded  in  producing  a 
large  series  of  compounds  analogous  to  sodahte,  in  which  the  original 
silicate  had  taken  up,  at  moderately  high  degrees  of  heat  and  pressure, 
various  other  salts  of  sodium,  such  as  the  chlorate,  selenate,  formate, 
oxalate,  and  so  on.  These  compounds,  however,  are  all  hydrated, 
and  so  differ  from  the  natural  minerals  of  the  sodalite  group,  and  they 
are  regarded  by  Thugutt  as  formed  by  molecular  union.  Following 
Lemberg,  he  regards  sodalite  as  a  molecular  compound  of  nephelite 
with  sodium  chloride,  and  taking  his  series  of  compounds  throughout, 
he  looks  upon  the  sodium  salts  which  have  been  added  to  the  funda^ 
mental  sihcate  as  equivalent  in  function  to  water  of  crystaUization. 
In  favor  of  this  view  he  cites  many  arguments,  some  of  which  are 
entitled  to  considerable  weight.  Thus,  when  sodahte  is  ignited 
NaCl  is  driven  off,  whereas  if  the  chlorine  were  united  with  aluminum 
AICI3  should  be  expelled.  Similarly,  by  the  action  of  water  alone, 
sodium  chloride  can  be  spht  off  from  the  sodahte  molecule,  thus  indi- 
cating a  looser  form  of  union  than  the  proposed  structural  formulae 
show. 

But  what  is  molecular  union  ?  To  this  question  there  is  no  satisfac- 
tory answer,  and  even  in  the  case  of  water  of  crystallization  the  term 
is  only  a  confession  of  ignorance.  Unless  we  assume  the  existence  of 
two  kinds  of  chemical  union,  it  means  merely  that  the  structural  link- 
ing is  unknown,  and  that  the  problem  is  laid  on  one  side,  conveniently 
labeled  for  future  reference.  The  constitutional  formulae  here  adopted 
for  sodalite  and  cancrinite  are  intended  to  give  a  provisional  solution 
of  the  problem  in  their  particular  cases  and  to  express  the  genetic 
relationships  with  nephehte  on  the  one  hand  and  the  crystallographic 
analogy  with  garnet  on  the  other.  The  objections  to  them  raised  by 
Thugutt  are  serious  but  not  absolutely  conclusive.  When  sodium 
chloride  is  spht  off  from  sodalite  the  mechanism  of  the  reaction  is 

1  Deutsche  geol.  Gesell.  Zeitschr.,  1885,  p.  962. 

2  Mineralchemische  Studien,  Dorpat,  1891.  See  also  Thugutt  on  cancrinite,  Neues  Jahrb.,  1911,  p.  25. 
Zambonini  (Contributo  alio  studio  del  silicati  idrati,  Napoli,  1908)  regards  cancrinite  as  a  mixtiu'e  of  silicates, 
with  all  the  water  extraneous— that  is,  not  essential  to  the  molecules.  On  the  sodalite  group  see  also  Silvia 
Hillebrand,  K.  Akad,  Wiss,  Wien  Sitzungsber.,  vol.  119,  p.  775, 1910. 


THE   SILICATES  OF  ALUMINUM.  29 

quite  unknown,  and  the  relative  affinities  in  the  molecule  are  quite 
unstudied.  Until  these  are  understood  the  objections  raised  by 
Lemberg,  Thugutt,  and  others  are  not  fatal.  Furthermore,  the  pres- 
ence of  a  group  =A1 — CI  does  not  imply,  as  Thugutt  supposes,  the 
splitting  off  of  AICI3  by  heat.  To  effect  such  a  decomposition  three 
molecules  of  sodaUte  would  have  to  be  broken  up,  and  there  is  no 
probability  that  such  a  disintegration  would  occur.  At  all  events  the 
formulae  proposed  fulfill  a  definite  purpose,  even  though  they  are  not 
finally  established.  They  express  known  relations  but  not  necessarily 
all  the  relations  which  the  future  may  reveal.  The  facts  that  the 
sodalite-cancrinite  minerals  are  derivable  from  nephelite  and  that 
nephelite  is  again  derivable  from  them  are  unquestionable. 

The  question  of  the  molecular  structure  of  a  typical  garnet, 
Al2(Si04)3Ca3,  remains  to  be  considered.^  If  it  is  regarded  as  a 
derivative  of  the  normal  salt  Al4(Si04)3  it  may  be  written  in  at 
least  two  ways,  thus: 

1.  2. 

.SiO,^l  SiO,=Ca 

Ai:lsio,^r^^  ^\siO,=Ca 


\siO,^l  ^SsiO,=Ca 


That  is,  isomerism  is  possible,  and  of  the  two  species,  partschinite  and 
spessartite,  one  may  belong  to  one  type  and  the  other  to  the  other 
In  the  first  expression  there  is  still  a  replaceable  atom  of  aluminum, 
but  in  the  second  expression  none;  in  the  first  at  least  one  calcium 
atom  must  link  two  SiO^  groups,  whereas  in  the  other  no  such  linkage 
occurs;  and  these  facts  may  be  connected  with  others.  For  example, 
garnet  alters  into  mica,  and  the  mica  group,  as  will  be  seen  later,  con- 
tains members  in  which  the  third  aluminum  atom  is  replaced.  This 
points  at  once  to  the  first  type  of  formula  as  preferable,  and  the 
alterability  of  garnet  into  epidote  brings  the  latter  mineral  into  the 
same  category. 

Zunyite  and  sodalite,  being  isometric,  should  also  foUow  garnet, 
but  derivatives  of  the  second  type  are  theoretically  possible  and  may 
exist.  Even  under  the  first  type  alone  isomerism  is  conceivable,  and 
the  orthorhombic  zoisite  may  be  contrasted  with  the  monoclinic  lime 
epidote  as  f oUows : 

Si04=Ca  SiO,=Al-OH 

/        >A10— H  /         >Ca 

Al— Si04==Ca  Al— Si04=€a 

\si04=Al  \siO,=Al 


» Tschermak  (K.  Akad.  Wiss.  Wien,  Sitzungsber .,  vol.  US,  p.  217, 1906)  regards  garnet,  epidote,  zoisite,  and 
prehnite  as  salts  of  the  acid  KShOs,  but  his  formula  do  not  well  show  the  relations  of  these  minerals  to 


other  species. 


30  THE   CONSTITUTION   OP   THE   NATUKAL  SILICATES. 

even  though  we  can  not  assign  either  species  to  either  formula  defi- 
nitely. My  obj  ect  here  is  merely  to  show  that  the  formulas  have  prop- 
erties by  virtue  of  which  they  are  able  to  express  known  differences. 
Additional  evidence  for  the  formula  assigned  to  garnet  is  supplied 
by  the  composition  of  vesuvianite,  which  is  most  simply  represented 
by  the  coalescence  of  two  garnet  molecules  with  partial  hydration, 
thus: 

Garnet.  Vesuvianite. 

SiO  =  1  SiO,=  1 

/  Cag  /  Caa 

Al— SiO,=  Al— Si04= 


\siO,=Al  \siO,=Al— OH 

Ca 
.Si04=Al— OH 


Al— 810^=  ] 

The  formula  agrees  well  with  many  analyses  of  vesuvianite,  but 
actually,  as  with  other  species,  its  composition  varies.  About  one- 
seventh  of  the  calcium  is  commonly  replaced  by  magnesium,  and  in 
some  varieties  boron,  presumably  in  the  group  =B — OH,  replaces 
in  part  the  corresponding  aluminum  radicle.^  Fluorine  is  also  often 
present  in  small  amount  as  the  equivalent  of  hydroxyl.  In  short,  a 
variety  of  isomorphous  replacements  or  comminglings  are  possible 
without  affecting  the  essential  structure  as  shown  by  the  formula. 
Such  replacements  are  too  well  known  to  need  detailed  discussion 
here. 

The  true  molecular  weights  of  silicates,  however,  are  unknown, 
and  it  is  therefore  conceivable  that  the  formulae  of  garnet  and  epidote 
should  be  doubled.  These  minerals  and  vesuvianite  have  nearly  the 
same  specific  gravities,  3.3  to  3.5,  for  the  purely  calcic  varieties. 
The  specific  gravities  in  the  cancrinite-sodalite  group,  on  the  other 
hand,  are  about  a  unit  lower,  a  fact  which  favors  the  simpler,  less- 
condensed  molecular  structure.  On  doubling  the  formulae  of  garnet 
and  epidote  the  following  comparison  with  vesuvianite  and  anorthite 
is  interesting: 

Garnet.  Anorthite. 

.Si04=Ca3=Si04v  .Si04=Ca3=Si04v 

Al— Si04=Ca3=Si04— Al  Al— SiO^^Al^^SiO^— Al 

\si0,^Al2=Si0/  \si04=Al2=Sio/ 

1  On  boron  in  vesuvianite  see  Wherry  and  Chapin,  Am.  Chem.  Soc.  Jour.,  vol.  30,  p.  1684, 1908.  Wein- 
garten  (Centralbl,  Mineralogie,  1902,  p.  726)  represent"?  the  mineral  by  the  formula  A10H=Si207=Ca2.  In 
the  former  edition  of  this  memoir,  U.  S.  Geol.  Survey  Bull.  125,  vesuvianite  was  given  the  formula 
Al2(Si04)6R6- AlOH.    See  also  U.  S.  Geol.  Survey  Bull.  262,  p.  72, 1906,  for  variations  in  the  present  formula. 


THE   SILICATES   OF   ALUMINUM.  31 

Epidote.  Vcsuvianite. 

^i04=Ca3=SiO,s^  ^SiO,=Ca3=SiO,v 

Al -SiO,=Al2=SiO,— Al  Al-SiO,=Ca3=SiO,— Al 

\siO,-  Ca-Sio/  \siO,-Ca  -Sio/ 

AlOH         AlOH  AlOH  AlOH 

In  these  new  formulae  the  essential  character  of  the  former  ex- 
pressions is  unchanged,  but  the  presumably  greater  condensation  is 
indicated,  with  the  derivation  from  two  molecules  of  aluminum 
orthosilicate  instead  of  from  one.  They  are  also  sustained  by  the 
facts  that  garnet,  epidote,  vesuvianite,  scapolites,  and  in  some  locaU- 
ties  anorthite  often  occur  in  hmestones  as  products  of  contact  meta- 
morphism;  that  vesuvianite  alters  into  garnet,  garnet  into  epidote 
and  scapolite,  and  that  all  four  minerals  alter  into  micas  and  the 
magnesian  varieties  into  chlorites  also.  The  species  are  connected 
constitutionally  and  genetically,  the  analogies  connecting  them  are 
remarkably  suggestive  and  complete,  and  the  formulae  here  proposed 
render  those  analogies  intelligible.  In  the  Swedish  '^mangan- 
idocrase"  a  salt  occurs  which  is  doubtless  the  vesuvianite  eqidvalent 
of  spessartite,  but  the  compound  in  a  pure  state  is  unknown. 

Kyanite,  an  isomer  of  andalusite,  but  of  much  higher  specific 
gravity,  may  perhaps  be  represented  as  a  basic  member  of  the  garnet 
series,  although  it  is  morphologically  very  different.     The  formula 

.SiO,=Al=Si04. 
Al— SiO,=Al2=SiO  — Al 

\sio,         sio/ 

(aIo)3  (A10)3 

expresses  this  relation  and  also  its  comparative  instabihty.  Both 
kyanite  and  andalusite,  at  very  high  temperatures,  are  transformed, 
with  disengagement  of  heat,  into  a  third  isomer,  siUimanite,^  which 
probably  has  the  simplest  formula  of  the  three.  This,  however,  is  so 
purely  hypothetical  that  it  would  be  useless  to  discuss  the  several 
species  further. 

Two  more  species,  meHlite  and  gehlenite,  which,  like  vesuvianite, 
are  tetragonal,  may  perhaps  be  best  considered  now.  Both  species 
are  very  variable  in  composition,  and  neither  seems  to  admit  of  one 
definite  formulation.  They  appear  to  be  mixed  silicates,  like  the 
intermediate  plagioclase  feldspars  and  scapoHtes,  but  the  end  inem- 
bers  of  each  series  are  difficult  to  identify.  An  artificial  sihcate 
recently  obtained  by  Shepherd  and  Rankin,^  in  the  Geophysical 

1  See  Vernadsky,  Soc.  Min.  Bull.,  vol.  12,  p.  447, 1889;  vol.  13,  p.  256, 1890. 

2  Private  communication. 


32 


THE   CONSTITUTION   OF  THE   NATURAL  SILICATES. 


Laboratory  of  the  Carnegie  Institution,  is  probably  one  end  of  the 
gehlenite  series  and  has  the  formula  Al2Ca2Si07,  which  is  that  of  a 
basic  metasilicate.     Structurally  it  can  be  written 


^^^^XAlO^Ca, 


in  which  the  basic  radicle  — ^Al<^     ^Ca  is  analogous  to  the  more 

familiar — ^A1=(0H)2.  The  other  end  of  the  series  is  probably  a 
silicate  of  the  vesuvianite  type, 

.SiO=Ca3=Si04v 
Al— SiO,=Ca3=SiO— Al 

\si04=Ca3=Si6/ 

in  which  the  three  replaceable  aluminum  atoms  of  the  normal  ortho- 
silicate  are  substituted  by  calcium.  If  this  supposition  is  correct  we 
have  the  following  complete  series: 

Ale(Si04)6Ca3,  anorthite.    . 
Al4(Si04)6Ca6,  garnet. 
Al3(Si04)6Ca9,  in  gehlenite. 

In  the  Mexican  gehlenite  analyzed  by  Wright  ^  there  is  an  approxima- 
tion to  Shepherd  and  Rankin's  silicate,  namely,  eight  molecules  of 
that  compound  commingled  with  one  of  the  other.  The  comparison 
is  as  follows: 


Found. 

Reduced. 

Calculated. 

SiOa 

26.33 

.03 

27.82 

L43 

.50 

.01 

2.44 

39.55 

.21 

.10 

1.85 

1           26. 69 

J 

1           29. 12 

44. 19 

26  51 

TiOg 

ALO, 

28  98 

Fe'a 

FeO.. 

MnO 

MgO 

CaO     .. 

44.51 

NajO                         .   . 

K2O    

HoO 

100.  27 

100.00 

100.  00 

The  second  column  is  recalculated  to  100  per  cent  after  uniting  iron 
with  alumina  and  recomputing  the  other  bases  to  their  equivalent  in 


1  Wright,  F.  E.,  Am.  Jour.  Sci.,  4th  ser.,  vol.  26,  p.  545, 


THE   SILICATES   OF   ALUMINUM. 


33 


lime.  Other  gehlenite  analyses  reduce  equally  well  but  with  much 
larger  proportions  of  the  orthosihcate  compound.  The  Orawitza 
gehlenite,  for  example,  is  very  nearly 

2  Al^da^SiO^  + 1  Al^da^Si A4. 

This  commingUng  of  an  orthosihcate  with  a  very  basic  metasiUcate  is 
not  easy  to  explain,  but  it  seems  to  fit  the  actual  evidence.  It  is 
furthermore  sustained  by  an  observation  of  Cathrein,^  who  has 
reported  pseudomorphs  after  gehlenite  of  fassaite,  a  metasiUcate, 
and  grossular  garnet.  The  formula  commonly  assigned  to  geh- 
lenite, Ca3Al2Si20io,  is  inadmissible. 

In  gehlenite  the  oxygen  is  always  in  excess  of  the  orthosihcate 
ratio,  but  in  melihte  the  reverse  is  generally  true.  The  Vesuvian 
melilites  agree  nearly  with  Groth's  formula,  Rg"  R2'''  Si fi^^,  but  the 
mineral  from  other  localities  exhibits  quite  different  ratios.  An 
artificial  ''mehlite"  obtained  by  Bodlander  ^  from  Portland  cement 
is  very  nearly 

^iO^=Ca3=Si04v 
Al— SiO,=Ca3  =SiO  — Al 

\si04=Mg3^SiO,/ 

as  the  following  comparison  shows : 


Found. 

Reduced. 

Calculated. 

SiOj                                                         --   - 

37.96 

9.46 

2.93 

12.77 

34.75 

1.53 

.64 

38.63 
1          1L51 

12.99 
1           36. 87 

39.22 

ALO,             '                             

11.11 

FegOg                                

MgO ....              

13.07 

CaO 

36.60 

K2O 

NajO                                                     

100.  04 

100. 00 

100. 00 

Other  melilites  seem  to  be  mixtures  of  this  type  of  compound  with 
the  corresponding  trisihcates — that  is,  with  SigOg  in  place  of  Si04, 
but  the  evidence  is  not  conclusive.  Such  mixtures  are  found  in  the 
feldspar,  scapohte,  and  mica  groups  and  are  well  known.  Just  as 
the  calcic  anorthite  crystaUizes  with  the  sodic  albite  so  probably  in 
melilite  two  compounds,  one  calcic  or  magnesian,  the  other  alkahne, 
replace   each  other  isomorphously.      Gehlenite   and  mehhte,   how- 


1  Min.  pet.  Mitt.,  vol.  8,  p.  408, 1886-87. 
43633°— Bull.  588—14 3 


2  Neues  Jahrb.,  1892,  vol.  1,  p.  53. 


34  THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 

ever,  seem  to  have  one  end  compound  in  common,  and  that  com- 
pound belongs  in  the  anorthite-garnet  series  as  already  shown.  Its 
synthesis  in  the  pure  state  is  yet  to  be  effected. 

Whether  the  formulae  here  proposed  are  true  or  not  they  are  useful 
for  purposes  of  correlation.  They  are,  moreover,  emphasized  by  an 
experiment  made  by  Lemberg,^  who  has  shown  that  gehlenite,  when 
heated  to  200°  with  a  solution  of  potassium  carbonate,  gives  calcium 
carbonate  and  a  product  having  the  composition  of  a  potash  mica, 
whereas  similar  treatment  with  sodium  carbonate  converts  the 
mineral  into  cancrinite.  Gehlenite,  garnet,  cancrinite,  and  musco- 
vite  are  therefore  related  to  one  another,  and  this  fact  is  expressed 
by  the  formulae  proposed.  Furthermore,  at  Orawitza,  in  the  Banat, 
gehlenite  is  found  in  rolled  pebbles  containing  grains  of  vesuvianite, 
a  fact  which  indicates  a  common  genesis  for  both  minerals. 

An  alternative  interpretation  of  the  relations  between  meUhte  and 
gehlenite  is  offered  by  the  hitherto  generally  accepted  theory  of 
Vogt,2  who  regards  both  species  as  varying  mixtures  of  two  silicates, 
one  the  *' gehlenite  silicate,"  Rj^'I^s^SiaOio,  and  the  other  akerman- 
ite,  R^'^SigOiQ.  The  last  compound  is  found  in  slags  and  has 
recently  been  identified  by  Zambonini  among  the  minerals  of  Vesu- 
vius. In  it  B,/^  is  principally  Ca,  but  with  a  notable  proportion  of 
Mg  also.  A  purely  calcic  siHcate  of  that  type  has  not  yet  been 
found.  Vogt's  theory  has  been  seriously  questioned  by  Bodlander 
and  Zambonini.  The  new  interpretation  now  offered  seems  to  be 
more  general. 

The  little-known  mineral  arctohte  is  possibly  another  member  of 
this  group,  with  affinities  toward  prehnite.  Its  composition  is  fairly 
expressed  by  the  formula 

Al^CSiOJeCaMgH^ 

which  is  that  of  prehnite  with  CaMg  in  place  of  Csl^.  The  integrity  of 
this  species,  however,  is  not  yet  fully  established. 

THE  FELDSPARS  AND  SCAPOLITES. 

Although  orthosilicic  and  trisilicic  acids  are  technically  distinct  and 
from  the  chemist's  point  of  view  should  be  studied  separately,  their  salts 
containing  aluminum  occur  in  such  a  variety  of  mixtures  that  in  sev- 
eral groups  of  minerals  the  two  acids  must  be  considered  as  mutually 
equivalent  and  their  compounds  discussed  together.  Two  such 
groups,  closely  allied,  are  the  feldspars  and  the  scapoHtes. 

For  each  of  these  groups  the  theory  developed  by  Tschermak  has 
met  with  general  acceptance.     In  the  case  of  the  feldspars,  Tschermak 

1  Deutsche  geol.  Gesell.  Zeitsch.,  p.  237, 1892.  2  Neues  Jahrb.,  1892,  vol.  2,  p.  73. 


THE   SILICATES   OF  ALUMINUM.  35 

was  undoubtedly  anticipated  in  great  part  by  Hunt,  Waltershausen, 
and  others,  but  to  him  full  recognition  is  due.  More  recently  it  has 
been  put  upon  a  thorough  quantitative  basis  by  the  synthetic  experi- 
ments of  Day  and  AUen.^  According  to  this  theory  the  trichnic 
plagioclase  feldspars  consist  of  albite,  AlNaSigOg,  and  anorthite, 
CaAlaSiPs,  wliich,  commingled  in  various  proportions,  give  the  inter- 
mediate oligoclase,  labradorite,  andesite,  and  so  on.  There  are  also 
the  trichnic  microcline  and  its  monochnic  equivalent,  orthoclase,  both 
represented  by  the  formula  AlKSigOg,  the  monochnic  barbierite, 
isomeric  with  albite,  and  the  recently  described  carnegieite,  or  soda 
anortliite,  already  mentioned  as  an  isomer  of  nephelme.  The  mineral 
celsian,  BaAlaSiaOg,  is  empirically  the  barium  equivalent  of  anorthite, 
but  it  is  monochnic  and  isomorphous  with  orthoclase.^  Hyalophane 
and  other  barium  feldspars  are  mixtures  of  orthoclase  and  celsian. 

The  exact  nature  of  the  isomeric  equivalencies  among  the  feldspars 
is  not  clear;  they  may  be  due  to  the  structure  of  the  salts  inde- 
pendently of  the  acids  which  they  represent,  or  to  isomerisms  among 
the  acids  themselves.  The  latter  possibihty  was  discussed  in  the 
section  on  the  silicic  acids  and  seems  to  be  the  more  probable,  at  leaj^t 
so  far  as  the  trisilicates  are  concerned,  but  for  present  purposes  the 
problem  may  be  left  outstanding.  In  the  discussion  later  of  the  spe- 
cies eudidymite  and  epididymite  the  question  of  isomeric  trisihcates 
wiU  be  considered. 

For  the  scapolite  series  Tschermak  has  elaborated  a  theory  wliich  is 
closely  parallel  to  that  of  the  feldspars.  These  tetragonal  minerals 
are  shown  to  be  most  easily  interpretable  as  mixtures  of  two  end 
compounds,  meionite,  AlgCa4Si6025,  and  mariahte,  Al3Na4Si9024Cl. 
Neither  end  compound  has  yet  been  found  in  nature  quite  free  from 
the  other,  but  the  variations  in  composition,  in  optical  character,  etc., 
are  all  accounted  for,  and  the  theory,  so  far  as  it  goes,  is  satisfactory. 
I  have  tentatively  examined  some  possible  alternative  hypotheses,  and 
none  of  them  fulfills  all  necessary  conditions  so  well  as  tliis  scheme  of 
Tschermak' s. 

Upon  studying  the  feldspars  and  scapoHtes  more  closely,  certain 
analogies  appear  other  than  those  indicated  by  the  parallehsm  of  the 
two  series.  Both  groups  of  minerals  are  easily  alterable,  and  both 
yield  kaolui  as  a  final  product  of  the  change.  Furthermore,  both  alter 
to  muscovite,  or  to  pinite,  which  is  only  an  unpure  pseudomorphous 
mica,  and  kaolin  crystallographically  has  close  relations  with  the  mica 
family.  Feldspars,  scapohtes,  muscovite,  and  kaolin  are  therefore 
presumably  connected,  and  the  structural  formulsB  of  the  minerals 
should  render  the  relationship  apparent. 

1  Am.  Jour.  Sci.,  4th  ser.,  vol.  19,  p.  93, 1905.    2  Strandmark,  Zeitschr.  Kryst.  Min.,  vol.  43,  p.  89, 1907. 


36  THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 

The  typical  empirical  formulae  to  be  considered  are  now  as  follows : 

Albite AlNaSigOg 

Anorthite AlgCaSigOg 

Meionite Al6Ca4Si6025 

Marialite Al3Na4Si9024Cl 

Muscovite AlgKHaSiaOia 

Kaolin Al2H4Si209 

For  muscovite  the  constitution  has  already  been  indicated,  and  this 
clue,  together  with  the  general  hypothesis  of  derivation  from  normal 
salts,  enables  us  to  correlate  all  six  of  the  formulae  given.  To  do  this 
it  is  necessary  to  triple  the  formulae  of  albite  and  anorthite,  and  we 
have  the  following  expressions: 

Albite.  Anorthite. 

ySigO^Nag  .Si04^Al2=Si04v 

Al— SigOs^Al  Al— SiO,=Al2=SiO— Al 

^SigOs^Al  \siO,=Ca3=Sio/ 

Marialite.  Meionite. 

Si303^Na,  /SiO,=Al,=SiO,^ 

/         )>A1— CI  Al— SiO,=Al2=SiO,— Al 

Al-Si^OsdNa,  \sio_Ca,=Sio/ 

\Si3O3SAl  I  J 

Ca O da 

Muscovite.  Kaolinite. 

^iO,=KH2  /OH 

Al-Si04=Al  Al— SiO,=H3 

\siO,=Al  ^SiO^^Al 

On  this  basis  mariahte  becomes  the  trisilicate  equivalent  of  soda- 
lite,  although  the  two  species  are  quite  unlike  in  form.  Anorthite 
is  the  calcium  salt  corresponding  to  nephelite,  which  is  also  alterable 
into  kaolin.  Again,  garnets  are  known  to  alter  into  feldspars  and 
scapolite,  and,  according  to  Brauns,  in  the  alteration  of  diabase, 
prehnite  and  epidote  are  sometimes  derived  from  anorthite.  .  These 
species,  therefore,  are  all  connected  by  numerous  cross  relations,  all 
emphasizing  one  another  and  pointing  to  a  community  of  molecular 
type.  So  far  the  formulae  are  highly  suggestive,  but  as  yet  they  do 
not  indicate  the  mechanism  of  the  reaction  by  which  a  trisilicate 
feldspar  breaks  down  into  kaolin,  and  they  need  development  in 
that  direction.  Tschermak,*  from  the  composition  of  the  siUcic  acids 
derivable  from  the  several  minerals,  infers  that  albite  is  a  salt  of  the 
acid  HsSigOy,  and  that  anorthite  is  a  basic  metasiHcate.     Himcmel- 

1  Zeitschr.  physikal,  Chemie,  vol.  53,  p.  349, 1905. 


THE   SILICATES   OF  ALUMINUM.  37 

bauer/  by  the  same  method,  makes  meionite  a  metasilicate  and 
mariahte  a  derivative  of  a  new  acid,  HioSigOjg.  Such  formulae  fail  to 
express  the  known  relations  of  the  minerals  at  all  clearly. 

Closely  aUied  to  the  feldspars  in  its  petrographic  relations  is  the 
isometric  mineral  leucite,  AlKSiaOg.  Empirically  it  seems  to  be  a 
metasihcate  and  is  commonly  so  regarded,  but  it  may  easily  be  con- 
ceived as  a  mixed  salt,  containing  orthosilicate  and  trisilicate  mole- 
cules. By  alteration  it  yields  orthoclase,  nephehte,  muscovite,  and 
kaolin,  and  the  pseudoleucite  of  Magnet  Cove  has  been  shown  by 
J.  F.  Wilhams  to  consist  of  orthoclase  and  elseolite  intimately  com- 
mingled.^ This  case  probably  represents  the  typical  breaking  up  of 
leucite,  the  formation  of  kaoHn  or  of  muscovite  in  other  instances 
being  due  to  secondary  reactions.  On  the  other  hand,  C.  and  G. 
FriedeP  have  obtained  leucite  synthetically  from  muscovite  as  a 
starting  point,  orthoclase  and  nephehte  being  produced  at  the  same 
time,  and  Lemberg,*  in  his  experiments,  has  transformed  leucite 
into  sanidine,  anorthite,  and  microsommite  and  also  into  andesine. 
In  a  later  paper  ^  Lemberg  describes^the  action  upon  various  sihcates 
of  the  salt  NagSiOg.SHjO,  at  200°  under  pressure,  kaolin,  albite, 
elseolite,  leucite,  and  analcite  all  yielding  a  siHcate-cancrinite  contain- 
ing SiOg  in  place  of  CO3.  These  facts  connect  the  several  species 
together,  but  to  their  explanation  the  empirical  expression  AIKSijOg 
gives  no  clue.  A  formula  for  leucite,  to  be  satisfactory,  must  be  a 
multiple  of  this,  and  several  such  multiples  fulfill  the  conditions  of 
the  problem. 

The  isometric  form  of  leucite  suggests  at  once  a  relation  with  the 
sodahte  group,  and  this  can  be  indicated  by  the  quadrupled  formula 
Al4K4Si8024.  We  then  have,  as  a  distinct  possibihty,  the  following 
series  of  molecules,  including  for  comparison  the  tetragonal  mariahte. 

Sodalite.  Marialite.  Leucite. 

/        >A1-C1  /         >A1-C1  /         >Al-SiO,=Al 

Al-SiO^^Na^  Al— SisOs^Na^  Al— SigOs^K^ 

\siO,=Al  \si308^Al  \si30«=Al 

On  this  basis  leucite  is  clearly  reduced  to  the  uniform  type  of  the 
minerals  to  which  it  is  apparently  related,  and  also  of  those  into 
which  it  alters,  but  the  formula  proposed  can  not  be  regarded  as  final. 
It  is  offered  here  only  as  a  first  approximation  toward  answering  the 
questions  which  are  suggested  and  is  therefore  subject  to  modifica- 
tion in  the  future. 


1  Sitzungsber.  K.  Akad.  WLss.  Wien.,  vol.  119,  p.  115,  1910. 

«  Arkansas  Geol.  Survey  Aim.  Kept.,  1890,  vol.  2,  pp.  267  et  seq. 

3  Soc.  min.  Bull.,  vol.  13,  p.  134,  1890. 

<  Deutsche  geol.  Gesell.  Zeitschr.,  1876,  pp.  611-615. 

6  Idem,  1885,  pp.  961,  962. 


38  THE   CONSTITUTION   OF    THE    NATURAL   SILICATES. 

Closely  allied  to  leucite  is  another  isometric  mineral,  analcite,  which 
empirically  has  the  composition  AlNaSi206.H20.  This  species  alters, 
like  leucite,  into  feldspar,  and  an  alteration  into  prehnite  is  also 
recorded.  Furthermore,  Lemberg  has  shown,  in  the  papers  already 
cited,  that  leucite,  by  the  action  of  soda  solutions,  may  be  transformed 
into  analcite,  and  that  analcite,  by  similar  treatment  with  potash, 
yields  leucite  again.  With  these  facts  in  view,  analcite  may  be 
written 

SiO^  =Na2 
/  >A1— SiO,=Al 

Al— SigOg^Na^  +4H2O 

^SigOs^Al 

exactly  equivalent  to  leucite.  That  the  water  is  entirely  nonconstitu- 
tional  has  been  shown  by  the  experiments  of  G.  Friedel,^  who  found 
that  it  could  be  expelled  continuously  without  change  in  the  crystal 
nucleus.  The  dehydrated  mineral,  moreover,  could  take  up  water 
again,  or  instead  of  water  various  vapors  and  gases.  These  sub- 
stances seem  to  be  occluded,  much  as  water  is  held  in  a  sponge,  only 
the  ratio  between  the  water  and  the  silicate  is  definitely  molecular. 
This  phenomenon  seems  to  be  peculiarly  characteristic  of  the  zeoUtes, 
which  will  be  considered  in  the  next  section  of  this  bulletin. 

The  close  relationship  between  leucite  and  analcite  is  shown  not 
only  by  the  work  of  Lemberg  but  also  by  investigations  carried  out 
in  the  laboratory  of  the  Geological  Survey  by  Clarke  and  Steiger.^ 
When  these  minerals  are  heated  with  dry  ammonium  chloride  in 
sealed  tubes  to  350°  C,  double  decomposition  takes  place  and 
ammonium  is  substituted  for  the  fixed  alkaUes.  In  each  case  the 
new  compound  has  the  empirical  formula  NH4AlSi206;  that  is,  an 
ammonium  analcite  or  leucite  precisely  equivalent  to  the  original  sili- 
cates is  formed.  A  volatile  base  has  replaced  the  fixed  bases,  and 
the  substance  so  formed  splits  up  on  ignition  in  such  a  way  as  to  shed 
light  on  its  constitution.  If,  now,  ammonium  leucite  is  a  true 
metasilicate,  a  salt  of  the  acid  HjSiOg,  it  should  break  up,  when 
ignited,  in  accordance  with  the  following  equation: 

2  NH,A1  (8103)2  =  Al2  (8103)3 +  2  NH3  +  H20  +  8i02 

and  one-fourth  of  the  silica  ought  to  be  set  free,  measurable  by 
extraction  with  sodium  carbonate  solution.  No  such  liberation  of 
silica  occurs,  and  we  may  therefore  conclude  that  analcite  and  leucite 
are  not  metasilicates,  but  more  probably  mixed  orthosiUcates  and 
trisilicates,  as  shown  in  the  constitutional  formulae  assigned  to  them 
here.  The  evidence  against  their  being  metasilicates  at  least  seems 
to  be  conclusive. 

1  Soc,  min.  Bull.,  vol.  19,  pp.  94,  363, 1896.  a  U.  S.  Geol.  Survey  Bull.  207, 1902. 


THE   SILICATES   OF  ALUMINUM.  39 

One  other  isometric  mineral,  pollucite,  may  perhaps  be  considered 
here.  Its  empirical  formula,  as  established  by  the  analyses  of  Wells  ^ 
and  Foote,2  is  that  of  a  metasihcate,  Hfis.Al^Sfi^,  which,  however, 
may  also  be  written  as  a  basic  trisilicate,  thus: 

^i30«=Cs,H 
Al— Si308=Cs2H 
\si30,=(A10)3 

If  further  investigations  should  show  that  the  water  of  pollucite 
is  not  an  essential  part  of  the  molecule,  its  empirical  formula  would  be 
very  close  to  AlCsSigOg,  and  the  mineral  would  become  the  caesium 
equivalent  of  leucite  and  analcite.  Pollucite,  however,  differs  from 
those  minerals  in  one  notable  respect,  namely,  on  heating  with  dry 
ammonium  chloride  only  one-third  of  its  caesium  is  replaced  by 
ammonium  instead  of  the  entire  amount.  This  observation  needs  to 
be  checked  by  experiments  on  pollucite  from  new  localities  before  any 
safe  conclusions  can  be  drawn  from  it.  That  pollucite  is  a  true 
metasilicate  is  very  doubtful. 

Although  kaohn  miner alogically  is  not  a  member  of  the  feldspar 
group,  it  is  properly  discussible  here  as  a  derivative.  The  formula 
assigned  to  it  in  the  foregoing  pages  is  not  unimpeachable,  but  it  sug- 
gests its  relations  to  the  feldspars  and  micas  and  also  represents  the 
fact  that  the  water  in  it  is  wholly  constitutional.  In  fact  the  mineral 
is  stable  far  above  the  ordinary  temperatures  of  dehydration,  so  that 
the  water  can  he  regarded  only  as  an  essential  part  of  the  molecule. 

In  addition  to  the  formula  proposed  for  kaohn  the  following  expres- 
sions are  possible  without  assumption  of  any  higher  molecular  weight: 

(1)  SiA.H4(AlO)3 

(2)  SiA-H2(A10H)2     (Brauns's) 

(3)  SiA.CAlHp^)^     (Groth's) 

Si03— AIHA 

(4)  H-0-Al<g.Q;_^ 

If  the  formula  be  tripled,  then  kaolin  may  be  written  as  a  basic 
trisilicate,  thus: 

(6)  Al— Si308=H.(AlHA)2 

\si30=(AlHA)3 

I  Am.  Jour.  Sci.,  3d  ser.,  vol.  41,  p.  213, 1891.  ^  Idem,  4th  ser.,  vol.  1,  p.  457, 1896. 


40  THE   CONSTITUTION    OF   THE   NATURAL   SILICATES. 

Of  all  these  symbols  only  the  last  and  the  one  originally  chosen  indi- 
cate the  relations  between  kaolin  and  its  parent  species.  As  for  these 
two,  the  formula 

yOH 

Al— SiO^^Hg 

\siO,=Al 

is  the  simpler  and  would  seem  to  represent  the  greater  stability. 
Kaolin  under  ordinary  circumstances  is  scarcely  attacked  by  the 
strongest  hydrochloric  acid,  a  fact  which  seems  to  be  most  in  har- 
mony with  the  orthosilicate  expression.  That  expression,  therefore, 
is  to  be  preferred,  at  least  until  more  positive  evidence  is  attainable. 
It  is  also  sustained  by  the  observation  of  Cornu  ^  that  kaolin  has  a 
faintly  acid  reaction  toward  litmus.  The  three  hydrogen  atoms  in 
union  with  the  Si04  group  suggest  such  an  acidity. 

After  dehydration  at  low  redness,  kaolin  is  completely  decom- 
posable by  hydrochloric  acid,  but  the  ignited  mass  contains  no  silica 
soluble  in  sodium  carbonate  solution.  These  facts,  developed  by 
experiments  made  under  my  direction  by  Mr.  George  Steiger,  seem  to 
indicate  the  formation  of  a  salt,  Al2Si207,  as  the  result  of  ignition,  but 
other  interpretations  are  possible.  The  data  are  given  here  simply 
as  data  that  may  become  available  for  a  fuller  discussion  of  the 
problem  by  and  by.  It  will  be  seen  later,  when  the  other  clays  are 
considered,  that  their  formulae  are  in  harmony  with  that  chosen  for 
kaolin. 

THE  ZEOLITES. 

By  this  title  is  indicated  a  well-defined  group  of  hydrous  sihcates, 
unmistakably  related  to  nephelite  and  the  feldspars.  Indeed  the 
relationship  is  so  close  that  the  several  species  can  often  be  studied 
genetically,  and  it  has  also  been  established  in  certain  cases  by 
synthetic  metjiods.  The  kinship  of  analcite,  which  is  commonly 
classed  as  a  zeoHte,  to  nephehte  and  leucite,  has  already  been  pointed 
out. 

For  example,  hydronepheUte,  natrohte,  and  analcite  all  occur  as 
alteration  products  of  nephehte  ;2  natrolite  and  analcite  are  both 
derivable  by  natural  processes  from  albite,^  and  analcite  yields  feld- 
spathic  pseudomorphs.  Natrolite  and  hydronephelite  may  be  gen- 
erated from  sodalite,  and  by  artificial  means  Doelter*  has  produced 
natrolite  and  analcite  from  nephelite.  All  these  relations,  with  others 
both  morphologic  and  genetic,  are  covered  by  the  types  of  formulae 
which  have  already  been  developed  and  which  can  be  extended  here. 

1  Min.  pet.  Mitt.,  vol.  24,  p.  417, 1905;  idem,  vol.  25,  p.  489,  1906. 

2  See  Brogger,  Zeitschr.  Kryst.  Min.,  vol.  16,  pp.  223  et  seq.,  1890. 

3  See  Brauns,  Neues  Jahrb.,  1892,  vol.  2,  p.  1. 
<  Neues  Jalirb.,  1890,  vol.  1,  p.  134. 


THE   SILICATES   OF  ALUMINUM.  41 

In  a  similar  way,  but  rather  less  completely,  many  zeolitic  minerals 
may  be  connected  with  anorthite,  the  calcium  end  of  the  plagioclase 
feldspar  series.  For  example,  by  heating  anorthite  with  freshly 
precipitated  silica  and  carbonic  acid  water  at  200°,  Doelter  obtained 
heulandite.^  Furthermore,  by  various  wet  reactions,  some  of  them 
unfortunately  involving  several  stages,  Lemberg  ^  has  generated 
analcite  from  chabazite,  gmelinite,  laumontite,  harmotome,  phillip- 
site,  stilbite,  and  heulandite,  for  some  of  these  minerals  studying 
several  varieties  of  one  species.  It  is  clear,  therefore,  that  the 
zeoUtes  are  connected  not  only  with  the  feldspars  but  also  with  one 
another  by  many  interlacing  relations  which  their  constitutional 
formulae  ought  to  symbolize.  These  relations  have  been  recognized 
by  all  modern  authorities,  but  their  interpretations  have  been  diverse. 

In  the  systematic  treatment  of  the  zeolites  the  most  serious  difficulty 
is  found  in  the  hydration  of  the  several  species.  To  determine  what 
part  of  the  water  in  any  one  of  these  minerals  is  constitutional  and 
what  is  crystalHne  is  not  easy,  and  no  fixed  criterion  exists  upon  which 
judgment  may  be  based.  At  present  the  weight  of  evidence  goes  to 
show  that  zeoUtic  water  is  extraneous  to  the  sihcate  molecule,  at  least 
so  far  as  suitable  experiments  have  been  made.  But  until  aU  zeoUtes 
have  been  studied  by  modern  methods  it  would  be  unwise  to  assume 
that  the  rule  is  universal.  Indeed,  in  some  zeolites  it  seems  probable 
that  constitutional  water  or  hydroxyl  is  actually  present,  for  on  no 
other  basis  are  the  analyses  easily  interpret  able.  The  work  of  Friedel 
on  analcite,  a  mineral  in  which  the  water  is  clearly  extraneous,  was 
cited  in  the  preceding  section  of  this  chapter,  and  analogous  researches 
have  been  conducted  by  other  investigators.^  Many  zeoUtes  lose  water, 
which  is  regained  without  much  change  of  crystalUne  character  on 
subsequent  exposure  of  the  minerals  to  moist  air,  and  this  water  at 
least  can  not  be  regarded  as  constitutional.  The  fundamental  fact, 
however,  that  zeoUtic  water  is  held  in  relatively  simple  molecular 
ratios  must  not  be  overlooked,  even  though  it  may  not  be  clearly 
explainable.  It  is  also  to  be  remembered  that  the  hydrated  silicates 
differ  in  crystalline  form  from  the  parent  anhydrous  minerals. 

In  the  former  edition  of  this  memoir  *  an  attempt  was  made  to 
discriminate  between  essential  and  nonessential  water  in  the  zeohtes, 
on  the  basis  of  various  researches  (Damour,  Hersch,  and  others)  rela- 
tive to  their  dehydration  at  successive  temperatures.  The  results 
obtained  were  instructive  and  of  some  significance,  but  the  modern 


1  Neues  Jahrb.,  1890,  vol.  1,  p.  128  et  seq. 

«  Deutsche  geol.  Gcsell.  Zeitschr.,  1885,  p.  959  et  seq.  ^     ,     ^.^        a    +«>.««  or,^ 

3  See  Rinne  (Neues  Jahrb.,  1896,  vol.  1,  p.  139;  idem.  1897,  vol.  1,  p.  41),  on  heulandite  and  stflbite^d 

Grandjean  (Soc.  min.  Bull.,  vol.  33,  p.  5, 1910)  on  the  replacement  of  zeolitic  ^^f/'^  ^t^^^^^f  ^^^"fj'P^": 

Zambonini  (Contributo  alio  studio  dei  silicati  idrati,  1908)  has  also  done  much  to  show  that  zeolitic  water 


is  essentially  absorptive. 
4  U.  S.  Geol.  Survey  Bull.  125, 1895. 


42  THE   CONSTITUTION    OF   THE    NATURAL   SILICATES. 

work,  as  just  cited,  renders  a- complete  revision  of  the  subject  neces- 
sary. Now,  regarding  water  as  not  belonging  to  the  true  silicate 
molecules,  we  may  discuss  the  zeolites  with  reference  to  their  genetic 
relations,  beginning  with  the  obviously  related  starting  points,  the 
formulae  of  nephelite  and  albite.  These  minerals,  as  we  have  already 
seen,  are  compounds  of  the  same  type,  one  an  orthosilicate,  the  other 
a  trisilicate,  Al3(Si04)3Na3  and  Al3(Si308)3Na3.  From  these  species 
hydronephelite,  natrolite,  scolecite,  mesolite,  analcite,  and  faujasite 
appear  to  be  derived,  either  directly  or  indirectly,  as  the  formulae  to 
be  proposed  clearly  show.  In  nearly  every  case  the  simplest  empirical 
formula  is  discarded  as  not  fairly  representing  the  known  relations 
between  the  minerals,  and  to  only  one  of  the  above-named  species 
does  the  rule  not  apply.  That  species  is  hydronephehte,^  an  obvious 
derivative  of  its  original  type  and  of  its  more  direct  parent,  sodalite. 
Its  formula  is  Al3(Si04)3Na2H.3H20. 

For  natrolite  alternative  formulae  have  been  proposed.  One, 
Al2(Si04)5Na2H4,  regards  the  water  of  the  mineral  as  constitutional. 
But  when  natrolite  is  heated  with  dry  ammonium  chloride  in  a  sealed 
tube  it  is  transformed  into  the  compound  Al2(NH4)2Si30io,  an  ortho tri- 
silicate. The  simplest  formula  for  natrolite,  then,  is  Al2Na2Si30io.  2H2O, 
which  is  not  obviously  related  to  nephelite  or  to  its  near  relatives 
among  the  zeolites.  Nephelite  can  be  written  as  a  basic  orthotri- 
silicate,  but  that  involves  more  difficulties  than  the  one  it  might 
seek  to  avoid.  Natrolite,  then,  with  other  species,  is  best  repre- 
sented in  a  less  immediately  obvious  manner  by  doubling  its  formula 
and  bringing  it, into  line  with  its  congeners,  especially  with  sodalite, 
scolecite,  mesolite,  and  edingtonite.  MesoHte,  however,  is  only  a 
crystalline  mixture  of  natrolite  and  scolecite  and  needs  little  consid- 
eration. Edingtonite  is  rather  doubtful,  but  Lemberg,^  by  the  action 
of  barium  chloride  solution  upon  natrolite,  obtained  a  silicate  which 
appears  to  be  that  mineral.  This  species  is  included  here  on  account 
of  its  chemical  analogy  to  scolecite.  So  much  premised,  the  formulae 
now  offered  are  as  follows,  beginning  with  the  anhydrous  type 
species: 

Nephelite.  Albite.  Sodalite. 

Si04=Na3  SigOg^Nag  Si04=Na2 

/  /  /        >A1-C1 

Al— SiO^sVl  Al— SigOgSVl  Al— Si04^Na2 

\si04^Al  \si3O3SU  \si04^Al 

1  Thugutt  (Neues  Jahrb.,  1910,  vol.  1,  p.  25)  regards  hydronephelite  as  a  mixture  of  iiatrolite,  gibbsite, 
and  diaspore;  but  the  miaeral  analyzed  by  me  was  purified  by  means  of  Thoulct  solution,  was  homogeneous 
under  the  microscope,  and  apparently  hexagonal  as  judged  from  its  optical  behavior. 

2  Deutsche  geol.  Gesell.  Zeitschr.,  vol.  28,  p.  553, 1876. 


THE   SILICATES  OF  ALUMINUM. 


43 


Hydronephelite. 

SiO.^Na^H 
Al— SiO.^Al 

3H2O 

Natrolite. 

/        >Al— SigOs^Al 
Al— SiO^^Na^ 

^SiO^^Al 
4H2O 

Edingtonite. 
SiO^^Ba 
/        >Al-Si30s=Al 
Al— SiO^^Ba 

\siO,=Al 

6H2O 


Analdte. 

SiO,=Na3 
,  /         >A1— SiO,=Al 

4H,0 

Scoledte. 
SiO^^Ca 
/         >A1— Si303=Al 

\siO,^Al 
6H2O 

Faujasite. 

SigOg^Nag 
/  >A1— SiO,sVl 

Al— SigOs^^Ca 

19HoO 


The  formula  assigned  to  faujasite  is  quite  unlike  that  usually  given, 
but  it  best  fits  Damour's  analysis,  as  the  subjoined  comparison  shows: 


Found. 

Calculated. 

SiOa 

46.12 

16.81 

4.79 

5.09 

27.02 

47  46 

ALO. 

16  14 

CaO 

4  43 

NaoO 

4.91 

HoO 

27.06 

99.83 

100.00 

The  isometric  character  of  faujasite  relates  it  to  analcite  and  leucite, 
but  its  immediate  derivation  was  probably  from  albite. 

A  group  of  monoclinic  zeolites,  closely  related  in  structure  to  the 
foregoing  species,  is  that  formed  by  wellsite,  phillipsite,  harmotome, 
and  stilbite,  to  which  Pratt  and  Foote  ^  assign  the  following  general- 
ized formulae: 

Wellsite : RAlaSigOio.SHaO 

Phillipsite RAl,Si40i2-4H20 

Harmotome - RAloSisOn-SHjO 

Stilbite RAlgSieOio.eHaO 

1  Am.  Jour.  Sci.,  4th  ser.,  vol.  3,  p.  443, 1897. 


44  THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 

These  formulae  make  a  beautifully  regular  series,  but  unfortunately 
they  do  not  represent  the  wide  variations  in  composition  exhibited 
by  some  of  the  species.  Harmotome,  a  barium  salt,  and  stilbite,  a 
calcium  compound,  are  fairly  constant,  except  for  variable  replace- 
ments of  the  dyad  radicle  by  sodium  or  potassium.  Wellsite  rests 
on  a  single  analysis,  in  which  calcium,  barium,  strontium,  potassium, 
and  sodium  appear.  In  phillipsite  R  is  principally  calcium,  but  with 
varjdng  replacements  by  potassium  and  sodium,  and  the  ratio  of 
silicon  to  oxygen  is  by  no  means  constant.  A  phillipsite  reported  by 
Zambonini  ^  is  very  near  R"Al2Si30io.4H20,  with  alkalies  partly 
replacing  lime.  Other  phillipsites  are  much  richer  in  silica  and 
approach  a  trisilicate  in  composition.  In  fact,  all  phillipsites  appear 
to  be  mixtures  of  orthosilicates  and  trisilicates,  ranging  between 
SSigOg  :  lSi04,  and  iSigOg  :  3Si04.  Such  a  range  and  even  a  greater 
one  is  only  to  be  expected  when  we  remember  that  many  zeolites  are 
derivatives  of  plagioclase  feldspars.  The  zeolites  vary  as  the  feld- 
spars vary  between  end  products,  which  may  or  may  not  be  definitely 
known.  Stilbite,  for  instance,  represents  a  hydrated  calcium  albite 
or  trisilicic  anorthite,  which  in  the  anhydrous  condition  is  yet  to  be 
discovered. 

In  this  series  of  silicates,  then,  we  have  the  plagioclase  variation  in 
the  ratio  Si  :  O,  whereas  the  ratios  R  :  Al  and  the  degree  of  hydration 
for  each  species  are  constant  or  nearly  so.  The  formulae  being 
qualified  by  recognizing  the  common  replacements  of  lime  or  barium 
by  alkalies,  the  four  ''species"  may  be  assigned  the  following  general 
expressions,  that  for  phillipsite  representing  a  fair  average  between 
its  extreme  variations : 

Wellsite.  Phillipsite. 

SiO^^R  Si04=Ca 

/         >A1— Si308=Al         /  >A1— SigOg^Al 

Al— SiO^^R  Al— SiO^MZJa 

^SiO^^Al  \si308=Al 

6H2O  8H2O. 

Harmotome.  Stilbite. 

SisOg^Ba  Sifi,=Csi 

/         >A1— SiO,=Al  /          >A1— SigOs^Ai 

Al— SisOs^Ba  Al— Si308=Ca 

\si308=Al  ^SisOg^Al 

IOH2O  12H2O 

In  phillipsite  as  much  as  half  the  calcium  may  be  replaced  by  potas- 
sium.    That  replacement  is  characteristic  of  the  species. 

1  Contributo  alio  studio  dei  silicati  idrati,  p.  114, 1908. 


THE   SILICATES   OF   ALUMINUM.  45 

The  hydration  of  these  silicates  increases  in  a  very  regular  and 
remarkable  manner,  and  proportionally  to  the  number  of  silicon  atoms 
in  the  molecule.  For  every  sihcon  atom  one  molecule  of  water  is 
retained.  This  rule  holds  true  for  the  typical  formulae,  but  if  incU- 
vidual  analyses  are  studied  considerable  variations  will  be  found. 
Phillipsite  nearly  always  contains  an  excess  of  water.  Moreover,  the 
low  molecular  weight  of  water  is  the  cause  of  apparent  irregularities 
when  formulae  are  deduced  from  analytical  data.  A  small  error  in 
the  determination  of  water  is  exaggerated  in  the  computed  ratios. 
The  rule  is  not  universal,  but  it  certainly  apphes  to  the  wellsite-stilbite 
series.  Stilbite,  at  one  end,  is  entirely  trisihcate;  at  the  other  end 
there  should  be  a  pure  orthosihcate  R2^l4(Si04)4.4H20,  but  no  such 
zeoUte  is  known.  The  orthorhombic  thomsonite  approaches  the 
required  composition  but  not  quite  closely  enough.  Empirically 
either  lawsonite  or  its  isomer  hibschite  would  complete  the  series,  but 
their  hydration  appears  to  be  constitutional,  and  crystallographicaUy 
they  belong  elsewhere. 

Among  the  plagioclase  zeolites,  if  such  a  term  is  admissible,  there 
are  two,  essentially  orthosihcates,  which  may  be  regarded  as  hydra  ted 
anorthite.  These  species  are  thomsonite  and  gismondite,  and  they 
may  be  represented  as  anorthite  plus  water,  using  the  tripled  formula 
for  anorthite  as  developed  in  the  preceding  section  of  this  work.  It 
is  better  perhaps  to  treat  them  as  less  condensed  molecularly  than 
anorthite,  because  of  the  loose  crystalline  structure  which  permits 
the  retention  of  zeolitic  water.  On  this  basis  their  formulae  fall  in 
line  with  those  of  the  other  zeoUtes,  as  follows: 

Thomsonite.  Gismondite.^ 

SiO,=Ca  SiO^^Ca 

/        Vl— SiO.^Al  /        \a1— SiO,=Al 

Al-SiO,^Ca  Al-SiO,^Ca 

\siO,=Al  \siO,-Al 

5H2O  SUfi 

In  each  of  these  minerals  variations  are  common,  just  as  among  the 
feldspars  from  which  they  are  probably  derived.  Gismondite  con- 
tains some  potassium  replacing  calcium,  or,  in  other  words,  an 
admixture  from  orthoclase  or  microcUne.  Thomsonite  may  have  as 
much  as  haK  its  Ume  replaced  by  soda,  due  perhaps  to  onginal 
carnegieite,  and  it  often  carries  an  excess  of  siUca,  either  in  ^'sohd 
solution"  or  else  representing  trisilicate  groups.  Carnegieite,  it 
should  be  remembered,  is  an  isomer  of  nephelite,  a  species  that,  under 
•  some  conditions,  alters  into  thomsonite. 

I  See  Zambonini  (Neues  Jahrb.,  1902.  vol.  2,  p.  79)  for  the  composition  of  gismondite  and  also  of  phiUipsite. 
See  also  Sachs,  Centralbl.  Mineralogie,  1904,  p.  215. 


46  THE    CONSTITUTION    OF    THE    NATURAL   SILICATES. 

Three  more  zeolites,  like  stilbite,  are  entirely  trisilicate,  namely, 
heulandite,  epistilbite,  and  brewsterite.  Heulandite  and  epistilbite 
are  isomeric,  or  in  crystallographic  terminology  the  compound  is 
dimorphous.  They  differ  from  stilbite  in  containing  less  water,  11 
molecules  instead  of  12.  The  formula  commonly  assigned  to  them, 
if  doubled  as  is  done  here,  assumes  only  10  molecules  of  water,  but 
all  trustworthy  analyses  give  a  larger  proportion.  Brewsterite  differs 
from  them  in  its  dyad  bases,  having  barium  and  strontium  w^ith  only 
a  httle  lime.  In  the  formula  to  follow  presently  the  calcium  is  united 
with  barium.  In  heulandite  strontium  is  often  present,  and  soda  to  a 
small  extent  replaces  hme.  The  two  formulae,  identical  in  type  with 
those  which  have  preceded  them,  are  as  foUows : 

Heulandite.  Brewsterite. 

SigOs^Ca  SigOg^-Ba 

/         >A1— SigOs^Al  /          >A1— SiaOg^Al 

Al— SigOs^Ca  Al— SigOs^Sr 

^SigOs^Al  ^SigOg^Al 

IIH2O  IOH2O 

Brewsterite  evidently  is  derived  from  an  unknown  feldspar  con- 
taining strontium.  Edingtonite  and  harmotome  may  represent 
original  celsian  or  hyalophane.  Doelter's  synthesis  of  heulandite 
from  anorthite  has  already  been  mentioned,  but  an  interesting 
observation  by  Rinne  ^  remains  to  be  noticed.  On  decomposing 
heulandite  with  sulphuric  acid  he  obtained  a  crystalline  form  of 
silica,  which  appeared  to  be  cristobalite.  This  fact  may  shed  some 
light  on  the  relative  molecular  magnitude  of  the  zeolite,  as  was 
suggested  in  Chapter  II,  on  the  silicic  acids. 

Baschieri,^  working  by  Tschermak's  method,  regards  heulandite 
and  stilbite  as  salts  of  an  acid,  HioSigOiy.  Natrolite  and  laumontite 
yielded  him  orthosilicic  acid,  and  analcite  he  formulates  as  a  dimeta- 
silicate. 

Erionite,  an  orthorhombic  zeolite  described  by  Eakle,^  probably 
belongs  as  a  trisilicate  with  stilbite  and  heulandite.  Its  formula, 
which  accurately  reflects  Eakle's  analysis,  is  as  follows: 

SigOs-^NaK 

/  >A1— SigOs^Al 

Al— SigOs^Ca 

^SigOs^Al 

I2H2O 

1  Neues  Jahrb.,  1896,  vol.  1,  p.  139. 

2  Zeitschr.  Kryst.  Min.,  vol.  46,  p.  479,  1909. 

3  Am.  Jour.  Sci.,  4th  ser.,  vol.  6,  p.  66, 1898. 


THE   SILICATES   OF  ALUMINUM.  47 

Laimiontite,  chabazite,  gmelinite,  and  levynite  are  plagioclase 
zeolites  in  which  the  ratios  are  empirically  metasilicate  or  nearly  so. 
That  is,  Si04  and  SigOg  groups  appear  in  equal  or  approximately 
equal  numbers.  Laumontite  is  essentially  calcic,  with  insignificant 
alkaline  replacements.  In  gmelinite  alkalies  predominate  and  lime 
is  quite  subordinate.  Chabazite  varies  widely  from  a  calcic  variety 
to  one  which  is  mainly  alkaline,  and  levynite  is  a  lime  zeolite  with 
Si04  to  SigOg  as  3  to  2.  The  variations  in  composition  are  quite 
like  those  which  occur  among  the  feldspars,  and  the  crystalUne 
comminglings  are  of  the  same  order.  If  regarded  as  a  zeolite,  anal- 
cite  is  of  similar  constitution  to  these  species  but  of  lower  hydration 
and  less  variabiUty.^  Lemberg's  syntheses  of  analcite  from  three  of 
them  have  already  been  mentioned. 

Now,  repeating  the  formula  of  analcite  to  facilitate  comparison, 
the  several  zeolites  can  be  well  represented  as  follows: 

Analcite.  Gmelinite. 

/         >Al-SiO,^Al  /         >Al-SiO,^Al 

Al— Si308=Na,  Al— SigOg^Na^ 

\Sig08^Al  ^SigOg^Al 

4HP  I2H3O 

Laumontite.            -.  ■       Caldum  chabadte. 

SiO.  ^Ca  SiO,  =Ca 

/         .>Al-SiO,sAl  /        >Al-SiO,=Al 

All-Si,0«^Ca  Al-Si30,^Ca 

\si30,=Al  \si30,=Al 

8H3O  12H,0 

Sodium  chabazite  is  empiricaUy  identical  with  gmelinite  but  different 
in  form.  This  sihcate  therefore  appears  to  be  dimorphous  or  isomeric, 
and  the  possibUity  of  isomerism  is  easily  shown.  One  acid  radicle  is 
represented  in  what  may  be  caUed  the  side  cham  of  the  molecule  as 
SiO,.  Let  that  exchange  places  with  an  SijO,  group  and  an  isomeric 
arrangement  is  at  once  given.  There  are  other  possibihties,  but  the 
one  is  enough  for  present  purposes.  As  for  the  last  of  these  plagio- 
clase zeohtes,  levynite,  the  best  analyses  represent  a  mixture  of  three 
orthosihcate  and  two  trisihcate  molecules  of  the  same  type  as  the 
other  members  of  the  group,  with  calcium  for  the  dyad  radicle,  and 
10  molecules  of  water.  The  formula  commonly  assigned  to  levynite, 
CaAl,Si30„.5H,0,  does  not  fit  the  facts.  From  Hillebrand  s  analysis 
of  the  Table  Mountain  levynite  we  get  the  foUowmg  companson 
between  observation  and  theory: . 

■  on  variation,  in  thocompositioaofanalclteseoroote and  Bradley,  Am.  Joar.  Soi.,  4th  ser.vol.  33, 
p.  433,  1912. 


48 


THE   CONSTITUTION    OF   THE   NATURAL   SILICATES. 


Found. 

Calculated. 

SiOa 

46.76 
21.91 
11.12 
1.34 
.21 
18.65 

46  55 

AloO, 

21  98 

Cab                 

]             12  07 

NagO 

KoO 

1 

HoO 

19  40 

99.99 

lOO! 00 

So  far  all  the  zeolites  considered,  except  hydronephelite,  are  assigned 
one  type  of  formula  with  varying  hydration.  If  we  unite  SiO^  and 
SigOg  under  the  indiscriminate  symbol  X,  the  general  formula  becomes 

and  this  covers  all  variations  of  composition  accurately.  R''  may 
stand  for  calcium,  strontium,  or  barium,  and  E,'  for  either  sodium  or 
potassium.  The  derivation  of  the  zeolites  from  feldspar  and  lenads, 
however,  is  not  always  direct.  In  many  cases  it  can  be  observed  and 
verified,  but  in  others  the  zeolites  seem  to  have  been  formed  in 
cooling  magmas  from  feldspathic  material  rather  than  from  the  feld- 
spars themselves.  Inclosed  bubbles  of  water,  perhaps  magmatic 
water,  have  helped  to  generate  the  zeolites,  especially  in  amygdaloid 
rocks.  The  zeolitic  amygdules  can  hardly  be  explained  otherwise, 
and  in  such  a  process  the  Si04  and  vSigOg  radicles  may  easily  be  sup- 
posed to  change  places,  forming  the  silicate  nuclei  corresponding  to 
calcium  albite  on  the  one  hand  and  sodium  anorthite  or  carnegieite 
on  the  other. 

Regardless  of  the  vahdity  on  nonvalidity  of  the  foregoing  sugges- 
tions, which,  by  the  way,  are  not  new,  the  constitutional  and  genetic 
connection  between  the  normal  zeohtes  and  the  feldspars  seems  to  be 
perfectly  clear,  and  it  ought  to  be  easily  confirmed  by  petrographic 
investigation.  Data  of  this  kind,  in  addition  to  those  alrec.dy  cited, 
are  even  now  available,  and  many  alterations  of  the  most  pertinent 
kind  have  been  observed.  Thus  laumontite,  heulandite,  stilbite,  and 
analcite  alter  into  albite  or  orthoclase;  laumontite  and  stilbite  into 
analcite ;  chabazite  into  natrolite ;  and  gismondite  into  phillipsite.  So 
also  alterations  into  prehnite  are  recorded  on  the  part  of  laumontite, 
scolecite,  mesolite,  natrolite,  and  analcite,  and  the  identity  of  chemical 
type  seems  to  be  ahnost  unquestionable.  From  the  formulae  here 
developed  all  these  alterations  become  intelligible,  and  the  theory  of 
substitution  from  normal  salts  is  very  emphatically  sustained. 

Several  other  zeolitic  minerals  are  known,  which,  however,  do  not 
belong  in  the  normal  series.     The  two  closely  allied  species  mordenite 


THE   SILICATES   OF  ALUMINUM.  49 

ajQd  ptilolite,  for  example,  are  to  be  classed  as  metadisnicates,  and 
their  constitution,  which  I  have  fully  discussed  elsewhere '  is  easily 
expressed  by  regarding  both  minerals  as  mixtures  of  the  two  molecules 

Al— S12O5  Al— Si^Os— R' 

ySi,0,         +6H2O     and        \si2O5  +6H3O 

Al— Si^O— H  Al— Si^Os— H 

\siA-H  \siA-H 

in  which  R^  =Na  or  K.  In  one  occurrence  of  ptilolite  the  water  is 
lower  than  is  required  by  these  formulae,  and  it  seems  probable  that 
a  trihydrate  may  exist. 

The  metasilicate  zeolite,  laubanite,  is  the  precise  equivalent  of 
ptilolite  and  mordenite  and  is  easily  interpreted  thus: 

Al— S1O3 
\siO3         +6H3O 

Al— SiOg 

\sio;>^^ 

Pilinite,  a  similar  mineral,  seems  to  be  Al2(Si03)5Ca2.H20,  a  monohy- 
drate  corresponding  to  the  hexhydrated  laubanite.  Unlike  laubanite, 
pilinite  is  undecomposed  by  hydrochloric  acid,  but  physically  all  four 
of  the  species  here  grouped  together  resemble  one  another  very  closely. 

Possibly  bavenite,2  CaaAlzSigOig.HaO,  is  to  be  classed  with  lau- 
banite as  a  metasilicate,  although  different  in  structure. 

Foresite  and  the  manganese  zeolite  ganophyllite  are  two  more 
species  of  unusual  form.     Their  formulae  can  be  written  thus: 

Foresite.  '  GaTwphyllite. 

Al--SiO,^Al  Al~-SiO,  =  J       ' 

\siO4=0a  +IOH3O        ^i04=    Mn     4-7H30 

Al-SiO,  =A1  Al-SiO,  =  I  ^^ 

\si308=Al  \si30s= 


1  Am.  Jour.  Sci.,  3d  ser.,  vol.  44,  p.  lOX,  1882. 

2  See  Artini,  R.  accad.  Llncei  Attl  rondicionti,  vol.  10,  p.  139, 1901. 


43633"— Bull.  588—1 


50 


THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 


These  expressions  represent  fairly  well  the  actual  composition  of 
the  two  minerals,  the  analysis  of  foresite  by  Manasse  ^  and  that  of 
ganophyllite  by  Hamberg  ^  being  taken  for  comparison. 


Foresite. 

GanophylUte. 

Found. 

Calculated. 

Found. 

Calculated. 

SiOa                           

48.93 
27.56 

49.91 

28.28 

39.67 
7.95 

.90 
Lll 

.20 

35.15 

2.18 

2.70 

.20 
9.79 

42  69 

ALO,          

}             8.60 

Fe!or              

CaO 

5.16 

5.18 

MffO    

MnO 

39.29 

NagO 

L14 

KoO 

PbO? 

HoO 

16.66 

16.63 

9.96 

99.45 

100.  00 

99.85 

100.00 

Both  species  need  additional  study,  especially  with  reference  to 
their  possible  variations. 

A  still  more  unusual  type  of  zeolite  is  stellerite,  recently  described 
by  Morozewicz.^  To  this  the  empirical  formula  CaAlgSiyOig.THaO  is 
assigned,  which  constitutionally  is  equivalent  to 

Al— Si^Os^       +7H2O 
\si308=Al 

Grattarola's  pseudonatrohte  seems  to  have  nearly  the  same  formula, 
but  with  only  SHjO.  The  structure  proposed  for  these  species  is,  of 
course,  only  tentative,  and  may  be  set  aside  at  some  future  time. 

A  considerable  number  of  other  zeolitic  minerals  have  been 
described,  but  their  nature  is  by  no  means  clear.  Offretite  may  be  a 
variety  of  phillipsite,  unusually  rich  in  potash,  and  gonnardite  may 
belong  with  natrolite  and  scolecite.  References  to  other  imperfectly 
known  zeolites  may  be  found  in  Dana's  Mineralogy. 

The  morphological  characteristics  of  the  zeolites  probably  depend  in 
great  part  upon  their  mode  of  hydration,  but  this  point  needs  to  be 
developed.  So  also  does  the  relation  between  zeohtes  and  kaolin,  into 
which  the  minerals  of  this  group  sometimes  alter.  Furthermore, 
zeolitic  substances  of  indeterminate  nature  are  believed  to  exist  in 
soils  and  clays,  and  it  is  conceivable  that  such  bodies  may  be  inter- 
mediately formed  during  the  transition  from  feldspar  into  kaolin.     In 


1  Zeitschr.  Kryst.  Min.,  vol.  35,  p.  514, 1902. 
a  Geol.  Foren.  Forh.,  vol.  12,  p.  586, 1891. 


Acad.  Cracovie  Bull.,  1909,  vol.  2,  p.  344. 


THE   SILICATES   OF   ALUMINUM.  51 

studying  the  mechanism  of  that  change  this  possibiUty  ought  to  be 
considered. 

THE  MICAS  AND  CHLORITES. 

On  account  of  their  wide  distribution,  their  variety  of  composition, 
and  their  genetic  relations  to  other  species,  the  micas  and  chlorites 
form  one  of  the  most  instructive  and  interestmg  families  of  minerals. 
Two  of  the  micas,  muscovite  and  biotite,  have  already  been  noted 
among  the  members  of  the  first  and  second  of  the  preceding  groups; 
and  we  have  seen  how  frequently  they  are  produced  by  the  alteration 
of  other  silicates,  some  of  which  have  been  synthetically  derived  from 
micaceous  material. 

As  regards  the  substitution  theory,  the  minerals  of  this  family  are 
pecuHarly  suggestive,  for  the  reason  that  they  form  a  series  of  the  most 
complete  character.  Thus,  starting  from  the  normal  aluminum  ortho- 
sihcate,  we  have 

Normal  orthosilicate AL(Si04)3 

Muscovite Al3(Si04)3KH2 

Normal  biotite Al2(Si04)3Mg2KH 

Normal  phlogopite Al(Si04)3Mg3KH2 

No  further  substitution  of  the  same  order  is  possible,  for  the  reason 
that  it  would  remove  the  Unking  atom  of  aluminum,  and  break  up 
the  fundamental  molecule. 

Muscovite,  the  first  species  in  the  foregoing  series,  occurs  in  nature 
as  an  independent  mineral,  and  also  as  an  alteration  product  of 
nephehte,  eucryptite,  topaz,  andalusite,  the  feldspars,  the  scapolites, 
and  various  other  natural  silicates.  All  these  alterations  become 
inteUigible  in  the  Hght  of  the  formulae  adopted  in  this  memoir.  In  its 
more  typical  occurrences  muscovite  agrees  sharply  with  the  formula 
given,  but  it  varies  in  composition  within  well-defined  limits.  First, 
it  ranges  toward  its  sodium  equivalent,  paragonite,  which  has  the  cor- 
responding formula  AlgCSiOJgNaHj.  Secondly,  in  fuchsite,  the 
chromic  mica,  a  chromium  salt  partly  replaces  the  aluminum  com- 
pound, and  similar  ferric  replacements  are  also  known.  The  chromic 
replacement  is  generally  quite  small,  and  so,  too,  is  that  of  iron, 
although  one  sericite  (a  secondary  muscovite),  analyzed  by  Senn- 
hofer,!  is  very  nearly  represented  by  AlaFeCSiOJgKHj.  Much  larger 
replacements  of  aluminum  by  vanadium  are  found  in  the  mineral 
roscoehte,  in  which  as  much  as  24  per  cent  of  V2O3  has  been 
determined.  An  ideal  roscoehte  should  have  the  formula  AIV2 
(SiOJgKHs,  requiring  33.6  per  cent  of  V2O3,  but  the  pure  compound 
is  yet  to  be  discovered.  In  kryptotile,  A^CSiOJaHg,  we  have  probably 
the  extreme  hydrogen  end  of  the  muscovite  series,  and  leverrierite 
may  be  the  same  species  but  of  different  origin.     The  presence  of 

1  Min.  pet.  Mitt.,  vol.  5,  p.  188, 1882-83;  Dana,  E.  S.,  System  of  mineralogy,  6th  ed.,  table,  p.  618, 1892, 
analysis  No.  43. 


52 


THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 


magnesia  or  of  ferrous  iron  in  a  muscovite  is  attributable  to  small 
admixtures  of  biotitic  molecules. 

The  most  important  variation  in  muscovite  is  in  the  direction  of 
increased  silica.  Normal  muscovite  contains  45.3  per  cent  of  SiO^j 
but  varieties  exist  in  which  the  percentage  rises  to  nearly  59.  Mus- 
covites of  this  class  have  been  designated  by  Tschermak  as  phengites, 
and  they  are  most  easily  explained  upon  the  supposition  of  trisilicate 
admixtures.  The  molecule  Al3(Si308)3KH2  is  identical  in  type  with 
ordinary  orthosilicate  muscovite,  and  its  presence  completely  accounts 
for  all  excesses  of  silica  over  the  normal  amount.  In  Sandberger's 
lepidomorphite,  for  instance,  the  orthosilicate  and  trisilicate  mole- 
cules occur  in  nearly  equal  proportions.  All  known  muscovite  may 
be  represented  by  the  general  formula  Al3(Si04)3E,'3  +  Al3(Si308)3R'3, 
in  which  the  latter  molecule  varies  from  0  to  50  per  cent,  and  with 
ferric  iron,  chromium,  or  vanadium  sometimes  partly  replacing  alumi- 
num. The  authenticity  of  this  trisihcic  variation  is  fully  confirmed 
by  certain  of  the  lithia  micas,  in  which  the  ratios  are  entirely  trisili- 
cate. AlUed  to  muscovite  and  paragonite  there  is  also  the  basic  mica 
euphyUite,  in  which  the  univalent  group  — A1=(0H)2  appears.  The 
formula  of  euphyllite  appears  to  be  Al3(Si04)3KH(A102H2),  which 
agrees  closely  with  the  best  analyses. 

With  the  biotites  and  phlogopites  the  variability  of  composition  is 
much  greater  than  in  the  muscovite  series.  Typical  or  normal  biotites 
may  be  represented  by  th^  subjoined  formulae,  the  actual  minerals, 
however,  being  commonly  mixtures. 


.SiO=MgK 
Al— SiO^^MgH 
\si04=Al 


.SiO=Fe''K 
Al— SiO,=Fe''H 
\siO,^Al 


.SiO,=MgK 

Al— SiO,^MgH 

\siO,=Fe''' 


.SiO,=Fe''K 
Al— SiO,=Fe''H 
\siO,=Fe''' 


These  formulae  correspond  to  the  following  c 

ompositions : 

1 

2 

3 

4 

SiOs 

43.06 
24.40 

37.  35 
21.16 

40.27 
11.41 
17.89 
17.89 

36.22 

ALO, 

9.98 

Fe^Oo                

15.66 

^                                   

19.14 

F&).;.;;.".. ..::::. :::::::: 

29.88 
9.75 
1.86 

28.18 

K2O 

11.25 
2.15 

10.52 
2.02 

9.20 

HoO 

1.76 

100.  00 

100.  00 

100.  00 

100.00 

THE   SILICATES   OF   ALUMINUM. 


53 


The  siderophyllite  of  Lewis  agrees  very  closely  with  No.  2  of  these 
formulae.  Haughtonite  is  near  an  equimolecular  mixture  of  Nos  1 
and  2,  with  some  ferric  replacement  of  aluminum.  Sodium,  as  in 
muscovite,  often  partly  replaces  potassium. 

In  the  normal  phlogopite  series  four  typical  compounds  may  occur 
but  the  entirely  magnesian  variety  is  the  only  one  which  is  found  even 
approximately  pure.     These  compounds  are — 


.SiO,=MgK 

Al— SiO,=MgH 

\siO,=MgH 

7 
/SiO,^MgK 

Al— SiO=Fe''H 

\siO,=Fe"H 


/SiO,=MgK 
AI— SiO,=MgH 
\si04=Fe"H 

8 
ySiO,=Fe''K 

Al— SiO,=Fe"H 
\siO,=Fe"H 


equivalent  to  the  following  percentage  compositions: 


5 

6 

7 

8 

SiOo 

43.27 
12.26 

28.85 

40.18 
1L38 
17.86 
16.07 
10.49 
4.02 

37.66 

10.67 

8.37 

29.70 

9.83 

3.77 

35.16 
9.96 

ALO, 

MgO 

FeO 

42  19 

KoO 

1L29 
4.33 

9  18 

HoO.. 

3  51 

100.  00 

100. 00 

100. 00 

100.00 

A  sodium  phlogopite  containing  no  potassium  has  been  described  by 
Griinhng/  but  his  analysis  is  not  altogether  satisfactory.  Aspidolite 
is  another  mineral  which  is  probably  sodium  phlogopite,  but  it  needs 
reexamination.  In  manganophyll,  a  manganese  mica  of  variable 
composition,  with  from  9.7  to  17.1  per  cent  of  MnO,  the  molecule 
Al(Si04)3Mn3KH2  seems  to  occur.  It  is  intimately  associated  with 
the  manganese  zeolite,  ganophylhte,  which  is  itself  micaceous  in 
appearance.     The  two  minerals  are  closely  related. 

To  the  typical  biotite  and  phlogopite  molecules  few  natural  micas 
actually  correspond,  although  intermediate  mixtures  are  very  com- 
mon. Many  of  the  analyses,  moreover,  are  difficult  to  interpret  with 
any  degree  of  accuracy  and  for  several  reasons.  The  state  of  oxida- 
tion of  the  iron  is  frequently  uncertain,  because  in  grinding  a  mineral 
for  analysis  ferrous  compounds  may  be  partly  oxidized  to  the  ferric 
condition.     In  fine  grinding,  furthermore,  some  water  is  adsorbed 

1  Zeitschr.  Kryst.  Min.,  vol.  33,  p.  218, 1910. 


54  THE   CONSTITUTION   OF   THE   NATURAL  SILICATES. 

from  the  atmosphere,  and  an  error  by  no  means  small  is  thereby 
incurred.  Titanium  is  also  present  in  many  micas,  and  its  exact 
function  in  them  is  quite  unknown.  It  may  be  present  as  TiOg 
replacing  silica,  as  TigOg  replacing  alumina,  or,  which  is  probably 
more  common,  as  inclusions  of  rutile.  An  unusual  type  of  mica  is 
Breithaupt's  alurgite,  which,  as  analyzed  by  Penfield,^  corresponds  to 
a  mixture  of  molecules — 

2Al3(Si303)3KH3 
SAl^CSiOJaK^H, 
3Al3(SiOj3Mg,KH 

with  a  slight  excess  of  H  over  K  in  the  last  compound.  The  second  of 
these  molecules,  an  alkaline  biotite,  is  the  characteristic  feature  of 
alurgite.  Similar  compounds,  parallel  to  phlogopite,  seem  also  to 
exist,  having  the  general  formula  Al (8104)311 'g,  but  all  of  these  bodies 
conform  sharply  to  the  general  theory  of  the  micas  and  are  substitu- 
tion derivatives  of  the  normal  aluminum  salt. 

In  many  of  the  magnesian  micas  fluorine  is  found,  and  the  iron 
micas  frequently  contain  oxygen  in  excess  of  the  amount  necessary  to 
convert  all  the  silicon  into  the  radicle  SiO^.  When  this  excess  is  real, 
that  is,  not  ascribable  to  defective  analysis,  it  may  be  due  either  to 
alteration  or  to  the  replacement  of  univalent  radicles  by  such  groups 
as  AIO2H2,  and  R^'OH.  Replacements  of  this  kind  indicate  a  transi- 
tion toward  the  chlorites,  as  will  be  seen  later. 

Fluorine  in  the  ferromagnesian  micas  may  represent  either  a  group 
like  — A1=F2  or  — R" — F,  and  these  appear  most  conspicuously  in 
the  lithia  micas.  An  average  lepidohte,  for  example,  agrees  well  with 
the  formula 

^Si303=(AlF2)3  ^SiO=H,K,Li 

Al— Si30=K3  +     Al— SiO=AI 

^SigOg^Lig  \siO=Al 

That  is,  the  mineral  is  a  mixture  of  a  trisiUcate  with  a  muscovite,  the 
actual  proportions  varying  on  both  sides  of  the  ratio  1:1.  Lepido- 
lite  sometimes  forms  borders  on  plates  of  muscovite,  and  Baumhauer  ^ 
has  shown  that  lepidohte  often  contains  inclusions  of  muscovite 
recognizable  only  under  the  microscope.  Another  Uthia  mica, 
zinnwaldite,  is  a  similar  mixture  of  the  same  trisilicate  with  a  ferrous 

biotite,  thus: 

/Si30s^(AlF3)3  ^SiO^^FeK 

Al— SigOg^Kg  +     Al— SiO  =FeH 

^SigOg^Lig  ^SiO^^Al 

1  Am.  Jour.  Sci.,  3d  ser.,  vol.  46,  p.  289, 1893.    Penfield's  interpretation  of  alurgite  is  quite  different  from 
that  adopted  here, 
a  Zeitschr.  Kryst.  Min.,  vol.  51,  p.  344, 1913. 


THE   SILICATES  OF  ALUMINUM. 


55 


which  represents  its  composition  very  closely.     The  calculated  com- 
position of  the  two  micas  is  as  follows : 


Lepidolite. 

Zinnwal- 
dite. 

SiOa 

5L43 
25.50 

46.54 
2L58 
10.15 
13.27 

3.17 
.63 

9,  04 

A1203 

FeO 

K2O 

13.43 

4.29 

.64 

8.13 

LioO 

HoO 

F 

Less  0 

103.  42 
3.42 

103.  38 
3  38 

^ 

100. 00 

100.00 

In  both  minerals  a  little  potassium  is  commonly  replaced  by 
sodium,  and  a  little  fluorine  by  hydroxyl.  Cryophyllite  is  near  zinn- 
waldite  but  more  complicated.  It  forms  borders  on  plates  of  the 
iron  mica  annite,  which  is  a  mixed  silicate  between  a  biotite  and  a 
phlogopite,  and  its  derived  lithia  mica  exhibits  similar  complexity. 
Other  known  lithia  micas  are  varying  mixtures  of  the  typical  mole- 
cules found  among  the  other  members  of  this  group  of  minerals,  and 
in  irvingite,  which  is  fully  two-thirds  trisilicate,  an  alkalLue  biotite 
appears.     Irvingite  is  well  represented  by  the  formula 

8Al(Si308)3(AlF2)3K3Li3  +  9Al2(Si308)3K2H,  + 1 1  Al^CSiOJgLigNag. 

Polylithionite,  which  is  entirely  a  trisilicate  mica,  has  a  quite  differ- 
ent type  of  formula  from  those  already  given.  The  typical  mineral 
as  shown  by  Lorenzen's  analysis  has  the  formula 

F  ^SieOs^Na^K 

5A1— F  +      lAl— Si303=Na2K 

\si308=Li3  \si308=Na3K 

which  leads  to  the  subjoined  comparison: 


AI2O3- 
FeO... 
K2O.-. 
NaaO-. 
LiaO.. 
F 

Less  O 


Found.       Calculated. 


59.25 

12.57 

.93 


102. 11 
3.08 


99.03 


59.79 
12.74 


5.85 
7.72 
9.34 

7.88 


103.  32 
3.32 


100.00 


56 


THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 


In  Flink's  analysis  of  polylithionite  from  another  locality  potassium 
is  in  excess  of  sodium,  but  otherwise  the  ratios  are  nearly  the  same. 

In  the  clintonite  group  or  so-called  ^'brittle  micas"  we  have  a 
series  of  highly  basic  compounds  commonly  free  from  alkahes.  They 
are  morphologically  like  biotite  and  are  characterized  by  the  presence 
of  the  univalent  radicle  AIO2R'',  R''  being  either  Mg,  Ca,  Fe'',  or 
Mn.  The  most  basic  mica  of  the  group,  the  end  member  of  the 
series,  is  xanthophyUite,  which  has  approximately  the  formula 
AlCSiOJgCAlOaR'Og,  with  Rg^MggCag.  In  seybertite  three  of  the 
univalent  radicles  are  replaced  by  hydrogen,  and  in  chloritoid  there  is 
still  more  replacement  of  a  different  kind.  The  ideal  formulae  are 
as  follows: 

Xanthophvllite.  Seybertite.  Chloritoid. 

^iO^— (A102Ca)3  ySiO,— H3  .SiO,=(A102H2)H2 

Al— Si04=(A102Mg)3      Al-Si04=(A102ll)3     Al— SiO,— (AIOH)H 
\si04=(A102Mg)3  \siO,=(A102R)3         \siO,=(A102Fe)3 

In  xanthophyUite  there  is  always  some  hydration,  and  in  the  other 
species  there  are  various  small  replacements  of  Al  by  Fe''',  of  Fe'' 
by  Mn,  etc.,  as  in  all  the  other  micas.  Ottrelite,  a  fourth  member 
of  the  series,  is  like  chloritoid,  but  contains  SigOg  instead  of  SiO^; 
that  is,  its  formula  is  trisihcate  The  pure  theoretical  compounds 
have  the  following  composition : 


Xantho- 
phyUite. 

Seybertite. 

Chloritoid. 

Ottrelite. 

SiOo    

16.45 
46.37 

21.53 
42.70 

23.81 
40.48 
28.57 

48  38 

AI2O3    

27  42 

FeO 

19.36 

MgO 

21.91 
15.27 

19.14 

13.40 

3.23 

CaO 

HoO 

7.14 

4.84 

100.  00 

100.00 

100. 00 

100. 00 

XanthophyUite  decomposes  on  ignition  into  two  portions — one 
soluble  and  the  other  insoluble  in  hydrochloric  acid.  The  insoluble 
portion  has  the  composition  of  spinel,  a  mineral  which  generally 
accompanies  cUntonite  micas,  and  of  which  the  formation  is  ren- 
dered inteUigible  by  the  formulae.^  The  actual  decomposition  may 
perhaps  be  represented  by  the  subjoined  equation: 


XanthophyUite.  Spinel.  Garnet. 

Ca3MgeAl,o(SiO,)30i3  =  4MgAl20,  +  Al,(SiO,)3Ca3  +  2MgO. 

1  For  a  different  interpretation  of  these  micas  see  the  former  edition  of  this  memoir,  U.  S.  Geol.  Sur- 
vey Bull.  125, 1895;  also  for  details  see  Clarke  and  Schneider,  U.  S.  Geol.  Survey  Bull.  113,  p.  27, 1893. 


THE   SILICATES   OF   ALUMINUM. 


57 


The  lime-alumina  garnet  and  the  free  magnesia  would  constitute 
the  soluble  portion  of  the  ignited  mineral.  The  equation,  of  course, 
is  purely  hypothetical  and  would  be  difficult  to  verify  experimentally. 
One  other  mineral,  willcoxite,  an  alteration  product  of  corundum, 
seems  to  be  best  classified  with  the  clintonite  micas.  It  appears  to 
be  a  basic  analogue  of  the  mixed  biotite-phlogopite  kind,  and  its 
analysis  gives  quite  sharply  the  subj  oined  formula : 


^i04=(A102Mg)2Na 
1  Al— SiO,=(A102Mg)2H 
\siO,=(A102Mg)3H 


^iO,=(A102Mg)Na2 
+     3  Al— SiO,=(A102Mg)H2 
\siO,=Al 


A  little  iron  is  present  in  the  mineral  and  the  sodium  is  partly  replaced 
by  potassium. 

From  some  points  of  view  kaolin  may  be  regarded  as  a  member 
of  the  mica  series,  especially  when  its  crystalline  form  is  considered. 
With  it  the  calcium  mica,  margarite,  which  is  commonly  classed  as  a 
member  of  the  clintonite  group,  can  be  conveniently  correlated. 
Furthermore,  margarite  yields  an  alteration  product,  dudleyite, 
which  falls  into  line  with  the  other  two  species,  thus : 

Kaolin.  Margarite.  Dudleyite. 

.OH  yOn  yOR 

Al_Si04=H3       Al— Si04=CaH       Al— SiO^^CaH 

\si04^Al  \si04=(A10)3        \siO,^A10H.AlHA 

Cookeite,  a  micaceous  mineral  found  associated  with  lepidolite  and 

encrusting  lithia  tourmalines,  also  seems  to  belong  here.     Its  formula 

is  simply  written 

•OH 

Al— Si04=Li,H(A102H2)  +  H3O 

\si04=Al 

which  agrees  well  with  Penfield's  analysis  of  the  cookeite  from  Maine 
and  Schaller's  analysis  of  the  California  mineral. 


SiOs- 

AI2O3 

Fe^Oa 

CaO. 

LijO. 

NaaO. 

K2O.. 

H2O.. 

F 


Penfield.        Schaller 


34.00 

45.06 

.45 

.04 

4.02 

.19 

.14 

14.96 

.46 


).32 


35.53 
44.23 


Trace. 

2.73 

2.11 

.31 

14.18 

L46 


100.55 


Calculated. 


35.08 
44.73 


4.40 
15.79 


100.00 


58  THE   CONSTITUTION   OF   THE   NATURAL  SILICATES. 

These  expressions  for  kaolin  and  its  analogues  are  suggestive  but 
not  altogether  conclusive.  They  represent  the  known  facts  fairly 
well,  however,  and  so  serve  their  purpose  for  the  time  being. 

By  hydration,  and  sometimes  by  oxidation,  the  micas  undergo 
alteration,  yielding  a  great  variety  of  products  which  are  known  in 
general  as  vermiculites.  This  is  especially  true  as  regards  the  ferro- 
magnesian  micas,  which  lose  alkalies  and  take  up  water  with  the 
greatest  ease,  in  accordance  with  what  seems  to  be  a  well-defined 
law.     Thus  we  have 

Biotite.  Hydrohiotite. 

^iO^^MgK  ^iO,=MgH 

Al— SiO,=MgH  Al— SiO^^^IgH  +  SH^O 

^SiO^^Al  \si04=Al 

Phlogopite.  Hydrophlogopite. 

^i04=MgK  ^iO,=MgH 

Al— SiO,=MgH  Al— Si04=MgH  +  SH^O 

\si04=MgH  \si04=MgH 

These  micas  occur  in  nature  in  great  variety  of  admixture,  and  the 
corresponding  vermiculites  show  a  parallel  complexity.  In  the  normal 
series,  however,  the  alteration  commonly  follows  the  line  indicated  by 
the  formulae,  and  the  vermiculite  is  simply  the  mica  with  H  in  place 
of  K  or  Na,  plus  3  molecules  of  loosely  combined  water.  Two  of  these 
molecules  are,  as  a  rule,  given  off  at  100°,  and  regained  in  moist  air, 
suggesting  an  analogy  between  the  vermiculites  and  the  zeolites. 
Some  vermiculites  are  only  monohydrated,  and  many  of  the  so-called 
species  which  have  received  names  are  mere  mixtures  of  altered  and 
unaltered  micas,  representing  stages  of  transition  between  the 
original  mineral  and  the  final  product.  Maconite,  lucasite,  and  phila- 
delphite  are  incompletely  altered  micas  of  this  kind. 

Jefferisite  is  quite  near  the  normal  hydrohiotite,  kerrite  approxi- 
mates to  a  hydrophlogopite,  and  lennilite  is  a  mixture  between  the 
two,  but  in  all  three  of  these  minerals  the  hydration  is  somewhat 
irregular,  and  there  are  the  usual  replacements  of  aluminum  and  mag- 
nesium by  ferric  and  ferrous  iron. 

In  some  of  the  vermiculites  basic  radicles  appear,  corresponding 
to  the  excesses  of  oxygen  over  the  normal  ratios  that  are  found 
among  the  micas  themselves.  For  example,  roseite  and  protover- 
miculite  may  be  written: 

Roseite.  Protovermiculite. 

.SiO,=(MgOH)2H  /SiO,=H3 

Al— Si04=(A10H)H  +  2H2O       Al— SiO,^(MgOH)3  +  SH^O 
\siO,=(A10H)H  \siO,^Al 


THE   SILICATES   OF   ALUMINUM.  59 

expressions  which  fit  the  actual  analyses  fairly  well.  Such  formulae, 
however,  must  be  interpreted  with  much  caution.  They  do  not 
necessarily  imply  that  these  micas  are  definite  compounds;  they 
merely  symbolize  one  kind  of  alteration  to  which  the  minerals  of  this 
group  are  subject.  They  mark  a  transition  between  the  micas  and 
the  chlorites,  and  similar  but  more  complex  examples  are  foimd  in 
hallite,  painterite,  pyrosclerite,  vaaHte,  and  pattersonite.  Cas- 
wellite  is  another  altered  mica,  rich  in  manganese  and  lime  but  of 
uncertain  formula.  Indeed,  it  is  hardly  worth  while  to  write  formulae 
for  these  minerals,  for  none  of  them  seems  to  be  a  single  definite 
compound. 

Between  the  micas  and  the  more  basic  chlorites  the  relations  are 
exceedingly  close.  All  the  species  are  foliated,  all  or  nearly  all  are 
monoclinic,  and  to  each  of  the  ferromagnesian  micas  one  or  more 
chlorites,  higher  in  magnesia  and  water,  seem  to  correspond.  The 
exact  formulation  of  the  chlorites,  however,  is  not  a  simple  matter. 
Some  of  the  so-called  '' species"  are  not  homogeneous;  others  are 
isomorphous  mixtures;  and  in  all  of  them  replacements  of  one  dyad 
base  by  another,  or  of  aluminum  by  iron,  occur.  In  the  chlorites 
the  basic  univalent  and  bivalent  groups  — Mg — OH,  — Fe" — OH, 
— A1=(0H)2,  =-Al— OH,  — Fe'''=(0H)2,  and  ==Fe"'— OH  appear, 
but  their  precise  identification  is  complicated  by  uncertainties  in  the 
hydration  of  the  minerals.  In  general,  the  water  shown  by  the  analyses 
is  constitutional,  but  in  some  chlorites  it  may  be  extraneous,  like  the 
water  of  the  zeolites.  An  exact  study  of  the  hydration  of  the  chlo- 
rites by  modem  methods  is  yet  to  be  made. 

Some  so-called  chlorites,  containing  alkalies,  are  obviously  mix- 
tures of  chlorites  and  micas,  but  the  true  species  are  all  referable  to 
the  types  of  molecule  represented  by  biotite  and  phlogopite  except  a 
few  that  fall  more  nearly  into  line  with  margarite  and  kaolin.  Typical 
chlorites,  which,  however,  are  rarely  if  ever  found  pure,  may  be  repre- 
sented thus: 

Biotite-chlorite.  Phlogopite-chlorite. 

.SiO,=(MgOH)2H  ^SiO,=(MgOH)2H 

Al— SiO,=(MgOH)2H  Al— SiO,=(MgOH)3H 

\siO,=Al  \siO,-(MgOH),H 

An  average  pennine  contains  these  two  molecules  commingled  in  the 
ratio  1:1,  whereas  in  clinochlore  and  leuchtenbergite  the  ratio  is 
2  :  3,  with  the  second  Al  of  the  biotite  formula  replaced  by  3A10. 
That  is,  the  formula  of  a  typical  clinochlore  is  represented  by 
3  Al(SiOj3(MgOH)eH3  +  2  Al(SiOj3(MgOH)A(A10)3. 
These  expressions  give  the  subjoined  compositions  for  the  two 
chlorites. 


60 


THE   CONSTITUTION    OF    THE    NATURAL   SILICATES. 


Pennine. 

CUno- 
chlore. 

SiOo 

34.35 
14.60 
38.17 
12.88 

31.  58 

AloO, 

19.  65 

MgO 

36.49 

H2O .   . 

12  28 

100.  00 

100.  00 

The  mixtures  may  occur  in  other  proportions;  the  magnesium  may 
be  partly  replaced  by  iron,  and  in  the  varieties  kammererite  and 
kotschubeite  some  chromium,  equivalent  to  aluminum,  appears. 
A  glance  at  the  tables  of  analyses  in  the  textbooks  of  Dana  and 
Hintze  will  show  how  variable  in  composition  these  and  other  chlo- 
rites  really  are. 

Three  chlorites,  rumpfite  and  the  ferrosoferric  minerals  cronstedtite 
and  melanolite,  seem  to  conform  very  nearly  to  the  biotite  type  of 
formula,  thus — 


Rumpfite. 


Cronstedtite. 


^SiO,^(MgOH),H  ^SiO=(Fe''OH)2(Fe'''0,H3) 


Al_SiO,=(A103H2)3 
\siO,=Al 


Fe— SiO,=(Fe"OH)2(Fe'''02H2) 

\siO=Fe'" 


Melanolite. 
.SiO,=(Fe''OH)H2 

Fe— SiO,=(Fe''OH)H2 

\siO,^Fe''' 

The  composition  of  the  three  minerals,  as  given  by  these  formulae,  is 
as  follows : 


Rumpfite. 

Cronstedt- 
ite. 

MelanoUte. 

SiOa               

30.20 

42.78 

20.93 

33.70 

ALOo 

fiK  :::. 

37.21 
33.48 

29.63 

FeO 

26.  67 

MgO                                                      .         .   - 

13.42 
13.60 

H2O                            

8.38 

10.00 

100.  00 

100.  00 

100.  00 

In  melanolite  a  little  ferric  oxide  is  replaced  by  alumina. 

The  two  species  brunsvigite  and  delessite  may  be  given  either 
biotitic  or  phlogopitic  formulae,  according  to  the  character  of  the 


THE  SILICATES  OP  ALUMINUM.  61 

hydration.  If  one  molecule  of  their  water  is  zeolitic  in  character- 
that  IS,  nonessential-the  minerals  are  of  the  biotite  type  If  all  th« 
water  is  constitutional,  they  are  related  to  phlogopite  Brunsvigite 
IS  essentiaUy  a  ferrous  chlorite,  delessite  is  ferromagnesian,  and  the 
two  species  correspond  to  the  two  formulae— 


Brunsvigite. 
^iO,=(FeOH)2H 

Al— SiO=(FeOH)2H 

\siO,=(A10H)H 


Delessite, 

^iO,=(MgOH),H 
Al-SiO,=(MgOH),H 
\siO  =(A10H)H 


In  delessite  the  magnesian  end  compound  is  sometimes  nearly 
approached,  but  usually  about  one-fourth  of  the  magnesium  is 
replaced  by  iron,  and  a  little  ferric  replacement  of  alumina  is  ako 
common.  Delessite  is  a  very  variable  mineral.  When  computed 
with  MggFei,  its  average  composition  is  as  follows,  m  comparison 
with  brunsvigite: 


Delessite. 

Brunsvig- 
ite. 

SiOs 

32.97 
18.68 
13.19 
21.98 
13.18 

28.04 
15.89 
44.86 

AloO, 

FeO 

MgO 

H2O 

ii.'2i 

100.00 

100.00 

Closely  related   to   the   two  preceding  species   are  prochlorite   and 
grochauite,  to  which  the  following  structures  may  be  assigned: 


Prochlorite. 

^iO,^(MgOH),H 
Al— SiO,=(FeOH)2H 
"^SiO^^CAlOH)  (AIO2H2) 


Grochauite. 

^iO,=(MgOH)3 
AI^SiO=(MgOH)3 
\siO,=(A10H)  (AIO2H2) 


The  composition  of  these  two  minerals  is  as  follows: 


Prochlorite. 

Grochauite. 

SiOo                                                        - 

28.28 
23.90 
22.50 
12.66 
12.  66 

27.53 

ALO, 

23.39 

FeO                                                                             

MgO 

36.69 

H2O                                                            

12.39 

100.00 

100.00 

62 


THE   CONSTITUTION    OF   THE    NATURAL   SILICATES. 


This  prochlorite  is  fairly  typical,  but  a  so-called  "  prochlorite " 
from  Culsagee,  North  Carolina/  is  quite  different  in  its  ratios  and 
contains  very  little  iron.  It  is  allied  to  metachlorite,  as  the  following 
formulae  show: 


Culsagee  prochlorite. 

^iO,=(MgOH)3 
Al— SiO,=(MgOH)2H 


\si04=(A10H)H 
The  percentage  compositions  are  as  follows : 


Metachlorite. 
^iO=(FeOH)3 

Al— SiO=(FeOH)2H 

\siO,=(A102H2)H 


Culsagee 
prochlorite. 

Metachlo- 
rite. 

SiOa 

28.48 
24.21 

22.73 

AloO, 

19.32 

FeO                         

45  46 

MgO              

3L65 
15.66 

H2O 

12.49 

100.00 

100.  00 

Corundophilite  is  a  chlorite  with  still  higher  alumina,  which  prob- 
ably has  the  following  constitution: 

^SiO,=(ROH)3 
Al~SiO=(ROH)3 
\siO,=(A10,H,)3 

SiO^ , 22.68 

AI2O2 '. 25.  69 

:^eO 17.88 

MgO 20.15 

^20 13.60 

100.  00 

The  calculated  composition  gives  Rg  =  Mg4Fe2.  The  water  is  a  little 
too  high,  and  may  be  partly  vermicuHtic.  If  so,  (A102H2)3  should  be 
replaced  by  (A10)3,  ^  replacement  which  may  be  desirable  in  some  of 
the  other  formulae  already  given.  The  need  of  investigating  the 
hydration  of  the  chlorites  has  already  been  pointed  out,  but  it  is 
well  to  emphasize  it  here. 

Diabantite   and   thuringite   appear  to   be  mixed  sihcates,   inter- 
mediate between  the  vermicuUtes  and  the  chlorites,  thus : 

1  Analyses  17-20  on  p.  654  of  Dana's  System  of  mineralogy,  6th  ed.,  1892. 


THE   SILICATES   OF  ALUMINUM. 


63 


1  Al— SiO,=RH 


Diabantite. 

^iO=(ROH),H 
+    2  Al— SiO,=(ROH)2H 
\siO,=RH 


Thuringite. 
^iO,=FeH  /SiO=(FeOH)3 

Al— SiO,=FeH  +     Al— SiO,=(FeOH)3 

\siO,=(A102H2)3  \siO,=.(A10,H,)3 

In  diabantite  Ri2  =  Mg7Fe5,  and  in  thuringite  one-third  of  the 
aluminum  is  replaced  by  ferric  iron.  Hence  the  following  composi- 
tions : 


Diabantite. 

Thuringite. 

SiOa 

34.93 
13.19 

22  78 

AloO, 

19  37 

FeoOo 

10  13 

FeO : 

23.29 
18.11 
10.48 

36.33 

MgO 

HoO 

1L39 

100.  00 

100.00 

The  talc-chlorite  of  Traversella  is  another  intermediate  compound 
simply  formulated  as  shown  below.     In  the  actual  mineral  one-fifth 

of  the  dyad  portion  is  Fe. 

.SiO,=(ROH)2H 

Al— SiO,=RH 

\siO,=RH 

SiOa ^ 39.00 

AlA 11.04 

FeO ^ 12.47 

MgO 27.75 

H2O 9.74 

100. 00 

Stilpnomelane  has  a  truly  chloritic  formula,  but  is  distinct  from 
the  others  in  being  a  trisihcate.  This  is  probably  true  also  of  the 
very  uncertain  ekmanite,  which  has  a  most  variable  composition. 
The  two  formulae  may  be  written  thus: 

Stilpnomelane.  Ehmanite. 

^i308=(FeOH)2H  ^iO,=(FeOH)3 

Al— Si30=(FeOH)2H  Al— SiO,=(FeOH)3 

\si,0,=(FeaH)3H  \siO,=(FeOH)3 


64  THE   CONSTITUTION   OF   THE    NATURAL   SILICATES. 

The  theoretical  composition  follows : 


Stilpnome- 
lane. 

Ekmanite. 

SiOa 

48.  91 
4.62 

39.13 
7.34 

40  91 

AloOo 

3  86 

FeO 

49  09 

HoO 

6  14 

100.  00 

100.  00 

Epichlorite  appears  to  be  a  mixture  of  stilpnomelane  with  the  equiv- 
alent magnesian  orthosilicate,  Al(Si04)3(MgOH)6H3,  in  nearly  equal 
percentages.  The  recently  described  minguetite  ^  is  apparently  a 
mixture  of  stilpnomelane  and  a  lepidomelane. 

So  far  all  the  chlorites  conform  to  the  mica  type  of  formula  as 
represented  by  biotite  and  phlogopite.  In  general  they  may  be 
regarded  as  salts  of  an  alumosilicic  acid,  Al(Si04)3H9,  and  its  corre- 
sponding trisilicate,  in  which  the  nine  hydrogen  atoms  are  replaceable 
by  a  variety  of  basic  radicles.  The  apparent  complexity  of  the 
chlorites  vanishes  and  the  relations  between  them  become  clear  and 
simple. 

There  is,  however,  a  group  of  chlorites  of  distinct  character  from 
the  normal  series.  Like  margarite  they  model  after  kaolin  in  the 
following  manner: 


Kaolin. 

Al— SiO^^Hg 

\siO,^Al 


Strigovite. 

/OH 
Al— SiO,=(Fe''OH)H2 
\siO,=Fe''' 


/ 


Aphrosiderite. 

OH 


Daphnite. 

yOR 

Al— Si04=(Fe''OH)3 
\siO,=(A10H)H 


Al— SiO,=(Fe''OH)3 
^SiO^^Al 

Sheridanite.^ 

.OH 
Al— SiO,^(MgOH)3 
\si04=(A10H)H 


The  theoretical  composition  of  the  four  chlorites  is  therefore  as 
follows : 


Strigovite. 

Aphrosid- 
mte. 

Daphnite. 

Sheridan- 
ite. 

SiO, 

33.  61 
14.17 
22.22 
20.00 

25.32 
2L52 

24.40 
20.73 

30.30 

ALOo 

25.76 

Fe'o'   ....     .. 

fS).....:  .. .;.:...:. .  :.:: 

45.57 

43.90 

MgO :. 

30.30 

H2O 

10.00 

7.59 

10.97 

13.64 

100.00 

100.  00 

100.00 

100.00 

1  Lacroix,  Soc.  min.  Bull.,  vol.  33,  p.  273,  1310. 

2  See  Wolff,  J.  E.,  Am.  Jour.  Sci.,  4th  ser.,  vol.  34,  p.  475, 1912. 


THE   SILICATES   OF  ALUMINUM.  65 

Strigovite  is  the  least  satisfactory  of  these  species,  for  the  reason 
that  the  analyses  are  discordant.  The  formula  given  it  in  Dana  may 
be  written  A10H(SiO,)2R.(ROH)3H,  which,  however,  is  of  the  same 
general  type  as  that  employed  here. 

Several  other  chloritic  minerals  have  received  names,  such  as 
chamosite,  klementite,  eurahte,  pycnochlorite,  epiphanite,  huUite, 
and  chlorophseite,  but  they  are  not  very  well  characterized.  The 
analyses  can  all  be  interpreted  in  harmony  with  the  other  chlorites 
but  it  is  hardly  worth  while  to  discuss  them  in  detail  until  the  several 
minerals  shall  have  been  more  thoroughly  studied.  Enough  has  been 
done  to  show  that  the  micas,  clintonites,  vermiculites,  and  chlorites 
form  one  systematic  group  of  minerals,  and  all  the  valid  evidence  is 
satisfied.  The  facts  that  garnet  and  vesuvianite  alter  into  chlorites 
and  that  chloritic  pseud omorphs  after  feldspar  are  known  serve  to 
connect  still  more  closely  the  formulae  here  adopted  with  the  similar 
formulae  of  the  preceding  groups  of  minerals. 

When  clinochlore  or  leuchtenbergite  is  strongly  ignited,  it  yields, 
like  xanthophylhte,  a  product  insoluble  in  hydrochloric  acid,  having 
the  composition  of  spinel.  This  reaction  establishes  still  more  defi- 
nitely the  relationship  between  the  chlorites  and  the  cHntonite  group, 
and  it  is  readily  intelUgible  in  the  light  of  the  structural  expressions. 
The  splitting  up,  under  influence  of  heat,  of  mixtures  containing  such 
groups  of  atoms  as  MgOH,  AlOH,  and  AIO2H2  ought  to  generate 
spinel,  and  the  appearance  of  a  compound  of  this  character  is  evidence 
in  favor  of  the  formulae. 

An  interpretation  of  the  chlorites  proposed  by  Tschermak  has  had 
some  acceptance,  but  the  scheme  is  complicated  and  subject  to  very 
serious  objections.  According  to  Tschermak  the  ''orthochlorites,'' 
which  include  pennine,  cHnochlore,  prochlorite,  and  corundophilite, 
are  molecular  or  crystalline  mixtures  of  serpentine,  H^MggSijOg,  and 
an  uncertain  substance,  amesite,  H4Mg2Al2Si09,  with  the  equivalent 
molecules  containing  iron.  Now  serpentine,  on  strong  calcination, 
breaks  up  into  water,  olivine,  and  enstatite,  the  enstatite  being 
insoluble  in  acids.  But  the  magnesian  chlorites,  which,  if  Tscher- 
mak's  theory  were  true,  should  yield  about  18  per  cent  of  enstatite  on 
ignition,  yield  none  at  all.  Spinel  is  formed  instead  of  enstatite,  and 
in  quantities  proportional  to  the  excesses  of  oxygen  over  the  ortho- 
siHcate  ratio.  That  is,  serpentine  molecules  are  not  present  in  the 
chlorites,  and  the  Tschermak  hypothesis  breaks  down. 

THE  ALUMINOUS  BOBOSILICATES. 

In  this  group  of  minerals,  of  which  tourmaline  is  the  most  impor- 
tant,  there  are  five  species,  namely,   tourmaline,   axinite,  dumor- 
tierite,    serendibite,    and    manandonite.     There    are    also    several 
43633°— Bull.  588—14 5 


66  THE   CON&TITUTION    OF    THE    NATURAL   SILICATES. 

borosilicates  of  the  rare  earths,  which  may  properly  be  studied  with 
them  even  though  they  contain  no  aluminum. 

Although  tourmaline  in  its  several  varieties  is  apparently  quite 
complex,  the  evidence  for  its  interpretation  is  abundant  and  ample. 
Its  variations  in  composition  are  shown  by  numerous  good  analyses, 
its  associations  are  well  known,  and  its  alteration  products  have  been 
observed  in  a  sufficient  number  of  examples.  From  the  minerals 
which  have  been  discussed  in  the  preceding  chapters  it  differs  essen- 
tially in  that  it  contains  boron,  and  the  part  played  by  this  element 
is  a  new  question  to  be  interpreted. 

When  tourmaline  undergoes  alteration,  the  commonest  product  is  a 
mica,  and  between  the  micas  and  the  tourmalines  there  are  very  strik- 
ing analogies.  With  the  lithia  micas,  Uthia  tourmalines  are  generally 
associated;  with  muscovite  and  biotite,  iron  tourmalines  occur;  and 
magnesian  tourmalines  accompany  phlogopite.  In  each  case  the 
composition  of  the  tourmaline  seems  to  bear  a  relation  to  that  of  the 
associated  mica.  Furthermore,  the  varieties  of  tourmaline  shade 
one  into  another  through  an  unbroken  series  of  gradations,  and  this 
may  happen  to  some  extent  in  one  and  the  same  crystal.  The  genus 
tourmaline,  in  short,  represents  a  series  of  compounds,  and  these  are 
parallel  to  the  normal  mica  series. 

Upon  the  constitution  of  tourmaline  there  have  been  many  essays 
written  and  much  controversy.  It  is  not  necessary  to  discuss  here 
the  Hterature  of  the  subject  in  detail,  for  only  two  types  of  formula 
are  now  seriously  considered,  and  they  differ  principally  in  regard  to 
the  ratio  between  silicon  and  oxgyen.  Penfield  and  Foote  ^  represent 
the  mineral  as  a  salt  of  the  acid  HuAlgEgSi^Oji,  which  may  be  written 

structurally  thus: 

^SiO,=H, 
Alf 

>SiO,=H2 


Al— B^Os^H, 


In  this  formula  the  boron  appears  as  the  radicle  of  an  acid  H4B2O5, 
which  is  a  rational  compound.  So  far  as  its  atomic  ratios  are  con- 
cerned, the  formula  fits  most  of  the  published  tourmaline  analyses 
very  weU,  although  not  aU  of  them,  but  it  does  not  suggest  the  well- 
known  alterability  of  the  mineral  into  mica.  It  is  also  difficult  to 
apply  in  detail,  that  is,  to  apportion  the  several  bases  replacing 
hydrogen  in  any  symmetrical  manner,  so  as  to  indicate  clearly  the 

1  Am.  Jour.  Sci.,  4th  ser.,  vol.  7,  p.  97, 1899.    Criticized  by  Clarke,  idem,  vol.  8,  p.  Ill,  1899.    Reply  by 
Penfield,  idem,  vol.  10,  p.  19, 1900. 


THE   SILICATES   OF  ALUMINUM.  67 

end  members  of  the  series  of  compounds  which  are  commingled  in 
tourmahne.  This  difficulty  has  been  avoided  in  part  by  later  writers, 
notably  by  Schaller'  and  Reiner,^  who  triple  the  Penfield-Footeformulai 
so  making  the  ultimate  tourmahne  acid  HgoSiiaBgOgg.  From  this  both 
Schaller  and  Reiner  derive  a  number  of  distinct  compounds,  which, 
as  mixed  crystals,  make  up  tourmahne.  The  tripled  formula,  how- 
ever, is  unwieldy,  and  not  easy  to  represent  by  a  simple  and  presum- 
ably stable  series  of  structures.  This  objection  is  entitled  to  some 
weight  but  is  not  necessarily  fatal. 

In  the  Penfield-Foote  type  of  formula  the  siUcon-boron-oxygen 
ratio  is  represented  by  Si^BPai,  which  is  equivalent  to  SigBgOgi.g.  In 
the  tourmahne  formulae  proposed  in  the  former  edition  of  this  memoir 
the  nucleus  SigBgOgi  appears,  with  an  oxygen  ratio  sHghtly  lower  than 
that  of  Penfield  and  Foote.  In  general  the  Penfield-Foote  formula 
fits  the  actual  analyses  a  little  better  than  mine,  but  the  difference  is 
very  slight  and  suggests  a  possible  constant  error.  Heretofore  all 
analyses  of  tourmahne  have  been  made  upon  finely  ground  material, 
and  fine  grinding  is  accompanied  by  more  or  less  adsorption  of  water 
from  the  air.  It  also  leads  to  some  oxidation  of  ferrous  iron,  a  change 
which  takes  place  very  easily  in  tourmaline,  and  these  two  almost 
certain  sources  of  error  tend  to  raise  the  oxygen  ratio  in  the  minerals 
as  analyzed.  These  sources  of  error  were  not  known  when  the  pub- 
lished analyses  were  made  and  must  now  be  taken  into  account. 

Now,  the  foregoing  considerations  being  kept  in  mind,  and  als<>  the 
alterabiUty  of  tourmaline  into  mica,  the  following  formula  for  the 
fundamental  tourmaline  acid  seems  to  be  probable: 

.SiO,=H3 
Al— Si04=H3 
\si04=Al— BO2 

Al— B03=H2 

ySiO^^Al— BO3 
Al— SiO,=H3 

This  involves  the  lowest  observed  ratio  between  aluminum  and 
silicon,  and  also  the  constant  ratio  between  sihcon  and  boron.  The 
boron  is  shown  as  partly  metaborate  and  partly  orthoborate,  which 
may  be  regarded  as  improbable.  As  an  alternative  the  boron  may 
be  represented  as  the  quinquivalent  group  Bfi,,  the  radicle  of  a  possi- 

1  Zeitschr.  Kryst,  Min.,  vol.  51,  p.  321, 1913.  ,:„hiflcl 

3  Inaugural  DLertation,  Heidelberg,  1913.    Reiner-gives  a  good  summary  of  earlier  work  on  the  subject. 

His  formijse  are  those  of  Wiilflng. 


68 


THE   CONSTITUTION   OF   THE   NATUEAL   SILICATES. 


ble  acid  H5B3O7,  a  rational  compound.  But  many  tourmalines  con- 
tain fluorine,  and  some  analyses  show  a  deficiency  of  boron,  which  is 
not  always  ascribable  to  analytical  error.  Fluorine  in  such  tourma- 
lines may  perhaps  replace  the  BO2  group,  a  supposition  for  which 
there  are  good  arguments,  although  it  may  not  be  absolutely  proved. 
To  this  point  reference  will  be  made  later. 

In  the  proposed  tourmaline  acid  various  replacements  of  hydrogen 
are  possible,  by  means  of  which  the  individual  tourmalines,  as  mixed 
crystals,  can  be  quite  accurately  formulated.  At  one  end  of  the 
series,  with  the  group  AlgSig,  we  find  some  magnesia  and  iron  tourma- 
lines, at  the  other  end,  approaching  the  ratio  AlgSi^,  are  the  colored 
lithia  tourmalines.  Of  the  lithia  tourmaHnes  the  extreme  member 
known  is  the  rubellite  from  Elba,  which  approximates  in  composition 
to  the  following  mixture  of  molecules : 


^iO,=Al 
Al— Si04=Al 
\siO,=Al— BO, 


^SiO,=H3  /SiO,=H3 

Al— Si04=Al  Al— SiO,=Al 

\siO,=Al— BO,  \siO,=Al— BO2 


3A1— BO,=Na,     +     4  Al— B0,=Na2    +    10  Al— BO,=Li, 


^iO,=Al— BO2 
Al— Si04=Al 
\si04=Al 


Al— SiO,=Al 
\si04=Al 


ySi04=Al— BO2 
Al— Si04=Al 
\siO— Al 


This  may  be  compared  with  Schaller's  analysis  of  the  mineral 
thus : 


Calculated. 


Si02. 

B2O3. 

T1203 

FeO- 
MnO. 
CaO. 
NagO 
LiaO. 
H2O. 
F... 


38.08 
11.06 
43.98 


100.  00 


The  differences  here  are  mainly  due  to  the  water  and  to  the  small 
neglected  impurities.     The  water  found  is  probably  too  high,  because 


1  All  the  analyses  of  tourmaline  cited  here  were  made  in  the  laboratory  of  the  United  States  Geological 
Survey. 


THE   SILICATES  OF  ALUMINUM. 


69 


of  fine  grinding.  The  comparison  is  also  influenced  by  the  fact  that 
the  mixed  molecules  are  not  exactly  represented  by  such  simple 
numbers  as  are  assumed  in  this  computation.  The  latter  considera- 
tion applies  throughout  the  discussion. 

At  the  other  end  of  the  series  we  have  the  magnesian  tourmaline 
from  Pierrepont,  New  York,  as  analyzed  by  Riggs.  The  mixed 
molecules  are  nearly  as  follows : 


SiO,=FeH 


Al— SiO^^FeH 
\si04=Al— BO 


'SiO,=MgH 


/SiO=MgH 
Al— SiO,=MgH 
\si04=Al— BOj 

3A1— B03=NaH      +     2  Al— BO,=NaH     +     5  Al— BO,=Ca 


Al— SiO,=MgH 
\siO,=Al— BO, 


xSiO^^Al— BO2 
Al— Si04=FeH 
\siO=FeH 


.SiO^^Al— BO2 
Al~SiO=MgH 
^SiO^^MgH 


/SiO,=Al— BO2 
Al— SiO,=MgH 
\si04=MgH 


Found. 

Calculated. 

SiOo 

35.61 

.55 

10.15 

25.29 

.44 

8.19 

1L07 

3.31 

L51 

.20 

3.34 

.27 

35.91 

TiOo 

B2O3 

10.48 

ALO,                                                                       

25.44 

FeoO,                                                  

FeO...            -.                 

8.62 

MgO 

n.i7 

CaO 

2.79 

NaoO 

L55 

KoO                                                             

HoO                                                   

4.04 

F                                                                                 

* 

99.93 

100.00 

An  interesting  intermediate  tourmahne  is  the  black  variety  from 
Lost  VaUey,  Cahfornia,  analyzed  by  Schaller.  Its  formula  is  that 
of  a  mixed  crystal,  as  follows : 

.SiO=Al  .SiO=FeH  ^iO,=MgH 

Al— SiO,=Al  Al— SiO=FeH  Al-SiO,=MgH 

\siO,=Al-BO,         \siO,=Al-B03         \siO,=Al-B03 

Al— BO,=H,      +     1  Al— B03=H2 


2A1— B0,=Na2    +     2  Al— BOe^H^      + 


/SiO,: 


=A1-B03         ^lo,=Al-B03         /SiO,=Al-BO, 
Al'_SiO=Al  Al-SiO,=FeH  Al-SiO=MgH 

\siO,=Al 


1         ^*v^4 

\siO,=FeH 


\siO,=MgH 


70 


THE    CONSTITUTION    OF    THE    NATURAL   SILICATES. 


Found. 


SiOa- 
B2O3. 
AI2O3 

Ti^Og 

FeO.. 
MnO. 
CaO.. 
MgO. 
Na^O. 
H2O.. 


35.96 

10.  61 

33.28 

.36 

1L04 

.13 

.42 

3.48 

2.16 

3.31 


100.  75 


Calculated. 


35.87 
10.42 
33.43 


1L43 


3.18 
2.46 
3.21 


100.  00 


It  is  not  necessary  to  give  more  examples  of  tourmaline  formulae, 
for  all  tourmalines  are  represented  by  the  type  of  structure  proposed 
here.  The  alkali  tourmalines  tend  toward  the  more  aluminous  end 
of  the  series,  the  magnesian  and  iron  tourmalines  toward  the  less 
aluminous.  All  the  tourmalines  now  known  are  evidently  mixed 
crystals,  but  the  complexity  of  the  mixtures  is  more  apparent  than 
real.  Whether  any  tourmalines  contain  ferric  iron  is  still  uncertain, 
but  the  probabilities  are  adverse  to  its  presence.  It  appears  in  many 
analyses,  but  that  is  probably  due  to  oxidation,  so  that  the  minerals 
analyzed  were  not  quite  normal.  Ferric  tourmalines  are  theoretically 
possible,  but  their  existence  is  unproved.  The  iron  of  tourmaline  is 
at  least  predominatingly  ferrous. 

The  formulae  for  tourmaline  adopted  here  not  only  express  the 
composition  of  the  mineral  but  also  indicate  its  obvious  relation  to  the 
micas,  and  its  ready  alterability  into  them.  A  molecule  of  tourmaline, 
with  elimination  of  boric  acid  and  One  atom  of  aluminum,  splits  into 
two  molecules  of  the  mica  type,  and  the  transformation  is  easily 
understood.  Potash  is  of  course  taken  up.  Certain  experiments  by 
Ijemberg,^  who  investigated  the  action  of  alkaline  solutions  upon 
tourmaline,  are  in  accord  with  these  suppositions. 

Although  otherwise  interpreted  by  Brogger,  the  minerals  cappelin- 
ite,  melanocerite,  karyocerite,  and  tritomite  seem  to  be  chemically 
akin  to  tourmaline.  This  view  of  their  nature  has  abeady  been  sug- 
gested by  Wiik,2  and  it  is  sustained  both  by  chemical  and  by  mor- 
phological considerations.  Cappelinite  is  hexagenal,  and  the  other 
species,  like  tourmaline,  are  rhombohedral.  They  are  silicates  of  rare 
earths,  which  are  mostly  trivalent,  like  aluminum ;  all  contain  boron, 
and  all  but  cappelinite  contain  fluorine  also.  Furthermore,  all  four 
species,  considered  together,  illustrate  the  reciprocity  between  boric 
acid  and  fluorine,  which  has  been  suggested  in  the  discussion  of 
tourmaline.  Thus,  if  we  compute  the  atomic  ratios  from  the  analyses 
cited  by  Brogger,^  the  following  relation  appears: 


1  Deutsche  geol.  Gesell.  Zeitschr.,  1892,  p.  239.  »  Zeitschr.  Kryst.  Min.,  vol.  16,  pp.  462-469, 1890. 

2Zeitschr.  Kryst.  Min.,  vol.  23,  pp.  421,  422, 1894. 


THE  SILICATES  OF  ALUMINUM. 


71 


Cappelinite . 
Melanocerite 
Karyocerite. 
Tritomite 


Si. 


236 
218 
216 
226 


488 

92 

134 

210 


304 

296 
226 


B+F. 


488 
396 
430 
436 


That  is,  Si  :  B  +  F  :  :  1  :  2  nearly,  variations  being  due  to  the  fact 
that  in  the  first  three  minerals  the  boric  acid  was  determined  by 
difference,  and  also,  probably,  to  the  occasional  replacement  of  fluorine 
by  hydroxyl.  Another  source  of  variation  is  found  in  the  presence  of 
tetrad  bases,  as  will  be  seen  later,  but  for  the  moment  the  relation 
indicated  seems  to  be  reasonably  clear. 

The  first  number  of  the  group,  cappelinite,  is  a  borosilicate  of 
yttrium  and  barium  and  approximates  in  composition  to 
/BO3  BO3 

Y— BO2  +4Y— BO2 

\siO,^BaH  \siO,=Y 

With  the  earths  of  uncertain  molecular  weight,  designated  as^'yttria," 
are  a  little  lanthanum  oxide  and  trifling  quantities  of  ThOg  and  CeOj, 
and  with  the  barium  are  some  calcium  and  alkalies. 

The  other  three  members  of  the  group  are  all  more  complicated  than 
cappelinite,  and  vary  from  it  in  t5rpe  by  containing  tetrad  oxides,  such 
as  CeOj,  ThOj,  and  ZrOg.  In  eudialyte  and  catapleiite  we  have  two 
rhombohedral  silicates  of  zirconia,  which  help  to  explain  these  com- 
pounds. Catapleiite  probably  has  the  constitution  (OH)3Zr.Si308.R'3. 
If  we  regard  the  tetrad  bases  in  the  cappelinite  group  as  forming 
orthosilicates  of  this  same  type,  the  remaincTer  of  each  mineral  may 
be  written  as  a  mixture  of  molecules  like  those  already  designated, 
but  with  cerium  earths  predominating  over  yttrium,  and  fluorine 
replacing  some  boric  radicles.  Thus,  melanocerite  is  not  far  from 
.OH  .B03=Ca  .F 


R^^< 


OH 


\si04=CaH  \siO,=R' 


-F 
\siO,=R' 


Karyocerite  may  be  written  similarly,  and  tritomite  becomes 
.OH  ^BO^ 

R^^<gg  +  R'"— BO, 

\siO,=CaH  \siO,=H,.R'"F3 

These  formulae  are  uncertain  and  need  verification  with  material  from 
other  sources.  At  present  they  have  only  a  reasonable  probabiUty. 
The  tetrad  silicate  in  them,  however,  will  be  seen  to  be  highly  prob- 
able when  we  come  to  the  discussion  of  the  other  allied  compounds 
in  their  proper  connection  later. 


72  THE   CONSTITUTION    OF   THE    NATURAL   SILICATES. 

The  second  aluminous  borosilicate,  axinite,  is  difficult  to  interpret 
structurally,  for  the  reason  that  its  relations  to  other  species  and  its 
modes  of  alteration  have  not  been  definitely  traced.  It  is,  however, 
easily  formulated  empirically  and  is  well  represented  as  a  varying 
mixture  of  two  molecules : 

Ferroaxinite HFeCa2Al2BSi40i6 

Manganaxinite HMnCa2Al2BSi40ia 

A  little  magnesia  is  also  shown  in  the  analyses  of  axinite,  which  points 
to  the  probable  existence  of  a  magnesium  salt  exactly  equivalent  to 
the  others.  A  distinctly  magnesian  axinite,  however,  has  not  yet 
been  found. ^ 

The  formulae  given  above  seem  to  make  axinite  an  orthosilicate, 
provided  that  the  boron  is  regarded  as  basic  and  equivalent  to 
aluminum.  But  boron  is  distinctly  an  acid-forming  element,  and 
therefore  it  is  more  probable  that  in  axinite  it  has  acid  functions. 
On  this  supposition  its  structural  formula  may  be  best  written  as  a 
mixed  orthosiHcate  and  trisilicate,  thus : 

B03=A1— OH 
Al— SiO^  =1 


R", 


Si.O. 


in  which  R"3=(Fe,  Mn)i  Gag,  the  calcium  being  constant.  Other 
structures  are  possible,  but  no  one  seems  to  have  any  advantage 
over  this.  Until  further  evidence  is  discovered  the  proposed  expres- 
sion may  be  regarded  as  vaHd,  but  it  represents  only  the  composition 
of  the  mineral,  and  nothing  more.  It  is,  however,  in  conformity  with 
the  general  theory  of  substitution. 

Dumortierite  is  much  more  easily  interpreted  than  axinite.  It  is 
related  to  andalusite,  with  which  it  is  often  associated,  and  it  alters 
into  muscovite.  Its  composition  has  been  determined  by  the  careful 
analyses  of  Ford  ^  and  SchaUer,  ^  and  is  simply  represented  by  a  sig- 
nificant constitutional  formula.  By  sHghtly  modifying  SchaUer's 
formula  the  following  comparison  is  obtained: 

Andalusite.  Muscovite.  Dumortierite. 

^i04=(A10)3  .Si04=KH2  .SiO^^AlO.BOH 

Al— Si04=Al  Al— Si04=Al         Al— Si04^(AlO)3 

\si04=Al  \si04=Al  \si04=(A10)3 

The  bivalent  group  =B — OH  is  evidently  equivalent  to  the  corre- 
sponding aluminum  group.  SchaUer  prefers  to  write  it  as  two  radi- 
cles, H  and  — B=0,  and  his  form  may  perhaps  be  better. 

1  For  a  full  discussion  of  the  composition  of  axinite  see  Schaller,  U.  S.  Geol.  Survey  Bull.  490,  p.  37,  1911. 
See  also  the  former  edition  of  this  memoir,  U.  S.  Geol.  Survey  Bull.  125, 1895. 
«  Am.  Jour.  Sci.,  4th  ser.,  vol  14,  p.  426, 1902. 
»  U.  S.  Geol.  Survey  Bull.  262,  p.  91, 1905. 


THE   SILICATES   OF  ALUMINUM.  73 

The  mineral  manandonite,  recently  described  by  Lacroix/  is  also 
easily  formulated.  As  written  by  Lacroix,  in  close  agreement  with 
the^  analysis,  its  empirical  formula  is  Hs^Li^Al^.B^SieOeg,  a  highly 
basic  compound.  Regarding  the  boron  as  present  in  the  group 
B4O7,  the  well-known  radicle  of  borax,  manandonite  may  be  given 
the  following  structure,  having  some  analogy  to  that  of  tourmaline: 

Al— SiO,=(A10A)3 
\siO,=Li2 

Al— B,0  — H 

/SiO,=Li3 
Al—SiO=iA\0,B:,), 
\siO,=(A10,H,)3 

This  formula  needs  to  be  confirmed  by  more  analyses,  and  an  inves- 
tigation as  to  the  relations  of  manandonite  to  other  species. 

One  other  borosihcate,  serendibite,  has  been  described  by  Prior 
and  Coomaraswamy,  ^  who  assign  to  it  the  empirical  formula 
10IlO.5Al2O3.B2O3.6SiO2,  in  which  RO  is  partly  CaO  and  partly  MgO, 
with  a  httle  FeO.  In  composition  it  has  some  analogy  to  tourmaline, 
and  it  also  resembles  the  ultrabasic  sihcate  sapphirine.  The  latter 
mineral  is  akin  to  the  brittle  micas,  in  which  the  univalent  group 

-Al/  NMg 


appears,  and  its  formula  may  be 

.0— A1=0 
Al— 0— A1=0 
\siO,=(A102Mg)3 

although  that  is  not  quite  certain.     With  these  clues  the  formula  of 
serendibite  may  be  written  thus: 

Al— Si04= ) 

Al— B205=(A10)3 

.   ^iO^^CAlO^Ca)^ 

AI— Si04=l 
\  Mg3 

SiO,=  J 


Soc.  Min.  Bull.,  vol.  35,  p.  223, 1912.  «  Mineralog.  Mag.,  vol.  13,  p.  224, 1903. 


74  THE   CONSTITUTION   OF   TPIE   NATURAL   SILICATES. 

which  fairly  satisfies  the  known  conditions.  This  formula,  however, 
like  some  of  the  others,  can  only  be  regarded  as  tentative. 

Although  it  contaiQs  no  alumina,  danburite,  CaBgSisOg,  is  perhaps 
most  conveniently  considered  here.  It  is  sometimes  regarded  as  the 
equivalent  of  barsowite,  CaAlaSigOg,  a  doubtful  isomer  of  anorthite. 
But  barsowite  gelatinizes  with  hydrochloric  acid,  whereas  danburite 
is  attacked  by  the  reagent  only  after  ignition.  Possibly  the  differ- 
ence may  be  as  follows: 

/SiO^^Al  /SiOg— B-=0 

Ca<  Ca< 

^SiO^sVl  \SiO3— B=0 

danburite  being  a  metasilicate.  Other  formulae  are  also  possible,  and 
more  data  are  needed  before  any  conclusion  can  be  reached.  The  asso- 
ciations of  danburite  with  feldspars,  mica,  pyroxene,  etc.,  suggest  that 
it  may  be  a  pseudometasilicate,  allied  in  structure  to  the  aluminous 
constituent  of  augite. 

MISCELLANEOUS  SPECIES. 

Among  the  orthosHicates  and  trisilicates  of  aluminum,  ferric  iron, 
and  other  triad  elements,  there  are  a  considerable  number  which  do 
not  fall  conveniently  into  any  of  the  preceding  groups  of  minerals,  or 
which  are  doubtful  as  regards  their  genetic  affinities.  Some  of  them 
have  obvious  relationships  to  other  species,  and  some  are  quite 
obscure  in  character,  but  all  seem  to  be  conformable  to  the  theory  of 
substitution. 

First  in  order  of  importance  is  the  mineral  staurohte,  a  liighly  basic 
siHcate,  which  is  evidently  akin  to  andalusite  and  sillimanite,  and 
which,  Hke  them,  is  orthorhombic.  Like  andalusite,  furthermore, 
staurolite  alters  to  muscovite,  an  entire  crystal  becoming  trans- 
formed throughout  into  an  aggregate  of  mica  scales. 

By  far  the  best  evidence  as  to  the  composition  of  staurohte  is  that 
furnished  by  the  analyses  of  Penfield  and  Pratt,^  who  adopt  Groth's 
formula,  HAlsFe^SigOig.     This,  structurally,  may  be  written 

/O— H 

Al— Si04=(A10)2 
\        >Fe 
SiO,=(A10)2 

which  expresses  a  partial  relation  to  the  micas,  andalusite,  and  so  on. 
The  theoretical  percentage  composition  calculated  from  this  formula 
agrees  well  with  the  results  of  analysis,  except  that  it  gives  the  sihca 
nearly  1  per  cent  too  low,  a  discrepancy  which  Penfield  and  Pratt 
attribute  to  inclusions  of  silica  in  the  minerals  analyzed. 

By  means  of  a  slightly  different  formula  the  relations  of  staurohte 
to  the  other  species  can  be  much  more  clearly  shown,  but  it  assumes 

1  Am.  Jour.  Sci.,  3d  ser.,  vol.  47,  p.  81, 1894. 


THE  SILICATES  OF  ALUMINUM. 


75 


that  the  ideal  staurolite  is  not  yet  known.     The  expressions  proposed 
are  as  follows: 

Andalusite.  Staurolite. 

/Si04=(A10)3  ^iO,=(A10)3 

Al— SiO,=Al  Al— SiO,=(A10)3 

\siO,=Al  \siO,=Fe 

Fe 

^iO^^Fe 
Al— SiO,=(A10)3 
\siO,=(A10)3 
This  formula,  in  contrast  with  that  of  Penfield  and  Pratt,  and  with 
their  reduced  analysis  of  staurolite  from  Lisbon,  New  Hampshire, 
gives  the  following  percentage  composition: 


Lisbon. 

Penfield 
and  Pratt. 

New 
formula. 

SiOs 

27.44 
55.16 
15.72 

L68 

26.32 
55.92 
15.79 

1.97 

27.90 
55  35 

ALO, 

Feb..    . 

16  75 

H2O...  . 

100.  00 

100.00 

100.00 

If,  now,  we  assume  that  the  actual  staurolite  is  slightly  altered  by 
hydration,  some  Fe  being  replaced  by  Hj  and  by  FeOH  +  H,  the  dis- 
crepancies between  formula  and  analyses  are  sufficiently  accounted 
for.  The  new  formula  is  more  symmetrical  than  the  old  one;  it  better 
expresses  the  alterability  of  staurolite  into  muscovite;  and  it  seems  to 
satisfy  the  evidence  with  sufficient  completeness.  When  we  remem- 
ber that  staurolite  is  excessively  liable  to  inclusions  and  alterations, 
a  very  sharp  agreement  between  analysis  and  theory  is  not  to  be 
expected. 

Still  another  orthorhombic  species,  harstigite,  has  a  formula  analo- 
gous in  some  ways  to  that  of  staurolite.  For  harstigite  there  is  but 
one  analysis  extant,  which  gives  nearly 

^i04=CaH 
Al— SiO,=CaH 
\siO,==Mn 

Ca 

Al— SiO,=CaH 
\siO,=CaH 


76 


THE   CONSTITUTION^  OF   THE  NATUBAL  SILICATES. 


This,  in  comparison  with  Flink's  analysis,  gives  the  following  per- 
centage composition: 


Found. 


Calculated. 


SiOo. 
AI2O3 
MnO. 
MgO. 
CaO. 
K2O. 
Na^O 
H2O. 


38.94 

39.13 

10.61 

11.09 

12.81 
3.27 

15.43 

29.23 

.35 

30.44 

.71 

3.97 

3.91 

99.89 


100.  00 


This  result  is  fairly  satisfactory.     More  data  relative  to  harstigite 
are  evidently  needed. 

Two  closely  related  silicates,  the  calcic  lawsonite  and  the  manga- 
nous  carpholite,  may  perhaps  be  analogous  in  structure  to  staurolite 
and  harstigite.  For  both  minerals  the  simplest  empirical  formula  is 
H^R^'AlgSijOio,  which,  tripled,  can  assume  the  following  form: 


Lawsonite. 

^i04=(A10A)3 
Al— Si04=H3 
\si04=Ca 

Ca 

^iO^^^Ca 

Al— SiO,=H3 

\si04=Al 


Carpholite. 

.SiO,=(A10A)3 
AI— Si04=H3 
\si04=Mn 


^iO,=Mn 
Al— Si04=H3 
\si04=Al 


Lawsonite  is  orthorhombic.  The  isometric  hibschite  seems  to  have 
the  same  composition,  but  is  too  incompletely  known  to  be  satis- 
factorily discussed  here. 

An  interesting  pair  of  ultrabasic  silicates  is  furnished  by  the  species 
kornerupine  (or  prismatine)  and  grandidierite.  Both  are  ortho- 
rhombic;  kornerupine  alters  into  kryptotile,  and  grandidierite  yields 
a  similar  and  perhaps  identical  derivative.  To  kornerupine  the 
empirical  formula  AlaMgSiOe  has  been  commonly  assigned,  which, 
tripled,  may  be  written: 

^iO,=(A10,Mg)3 
Al— SiO^^il 
\siO,^Al 


THE   SILICATES   OF  ALUMINUM. 


77 


in  close  analogy  to  andalusite  and  kryptotile.  The  recent  investi- 
gation of  prismatine  by  Uhlig/  however,  shows  that  the  mineral  is 
more  complex  and  in  better  agreement  with  the  formula 

NaEgMgeAl^^SiAo. 
Grandidierite,2  described  by  Lacroix  in  1902,  has  the  empirical 
formula  R'\(A1,  Fe)22Si7056,  with  aluminum  largely  predominant 
over  iron.  R"  is  mainly  magnesium,  but  with  some  replacement  by 
iron,  calcium,  and  a  little  sodium.  That  is,  it  is  a  mixed  crystalline 
mineral,  but  of  fairly  definite  type.  Both  species,  with  the  empirical 
formulae  now  given  them,  are  expressible  by  analogous  structures, 
thus  • 


Komerujnne. 

ySiO,^Al 
Al— SiO,=(A102Mg)3 

\si04=NaH 
Al— SiO^^Al 

'>SiO,=H3 
Al— Si04=(Al02Mg)3 

\si04=Al 


Grandidierite. 

^SiO,=(A102R)8 
Al— SiO,=(A10)3 

\siO,=(A10)2 
Al— SiO,=(A102R)3 

a1— Si04=(A10)3 
\siO,=(A102R)3 


The  nacreous  or  micaceous  mineral  batavite  ^  is  perhaps  of  similar 
type,  although  Weinschenk  has  assigned  it  the  relatively  simple 
formula,  4H20.4MgO.Al203.4Si02.  Its  composition,  however,  is 
equally  well  expressed  as  follows : 

^iO^^MgH 
Al— SiO,^MgH 

)>SiO,=Mg 
Al— Si04=MgH     +     4H2O 

\siO,=Mg 
Al— SiO,=MgH 
\si04^MgH 
Batavite,  like  many  other  minerals,  needs  further  investigation, 
as  the  following  comparison  between  analysis  and  formulae  clearly 
shows : 


Found. 

Calculated. 

Wein- 
schenk. 

New 
formula. 

RiO                                                 

42.33 
16.  35 
28.17 
13.19 

4L81 
17.77 
27.88 
12.54 

43.30 

A 1  0 

15.78 

MaO                                                            

28.86 

H20 

12.06 

100.04 

100. 00 

100.00 

1  Zeitschr.  Kryst.  Min.,  vol.  47,  p.  215, 1910. 

8  Soc.  min.  Bull.,  vol.  25,  p.  85, 1902;  vol.  27,  p.  259, 1904. 


78 


THE   CONSTITUTION   OF   THE   NATURAL  SILICATES. 


A  still  more  unusual  type  of  silicate  is  presented  by  didymolite,  a 
mineral  recently  described  by  Meister/  who  assigns  it  the  formula 
2Ca0.3Al203.9Si02.  Constitutionally  this  seems  to  be  a  basic  deriva- 
tive of  a  trisilicic  anorthite,  with  the  following  structure: 

.SigOg^AlO  (AlO^Ca)^ 
Al— SigOs^Al 
\si308=Al 

To  bityite,  a  mineral  from  the  tourmaline  region  of  Madagascar, 
described  by  Lacroix,^  may  be  assigned  the  empirical  formula 
H7Li2GlCa3Al9Sie034. 

This  leads  to  a  structural  formula  analogous  to  that  of  tourma- 
line, which  suggests  that  bityite  may  be  an  intermediate  compound 
between  tourmaline  and  cookeite. 

^iO,=(A10A)3 
Al— SiO^^I^iR" 
\siO,=R'' 

Al— OH 

^iO,=R" 
Al— SiO^^LiR'' 

\si04=(A10)3 

The  comparison  between  analysis  and  formula  is  as  follows: 


Found. 

Calculated. 

SiOo 

3L95 

4L75 

14.30 

.13 

2.27 

2.73 

.40 

.16 

6.50 

32^58 

ALO, 

4L54 

Cab.. 

1            15. 20 
2.26 

GIO 

LioO 

]              2.72 

NaoO 

KoO 

H2O 

5  70 

100. 19 

100. 00 

The  water  found  is  probably  a  little  too  high,  due  either  to  alteration 
or  perhaps  to  fine  grinding.  Tourmaline,  manandonite,  serendibite, 
and  bityite  seem  to  be  closely  related  species. 

The  vanadiosilicate  or  arseniosilicate,  ardennite,  is  a  compound  of 
quite  different  t5rpe  from  any  so  far  considered.     It  is  a  mixed  silicate 


Neues  Jahrb.,  1912,  vol.  1,  ref.  403. 


*  Soc.  min.  Bull.,  vol.  31,  p.  241, 


THE   SILICATES   OF  ALUMINUM.  79 

in  which  sometimes  the  vanadic  radicle  predominates  and  sometimes 
the  arsemcal  group,  but  the  structure  of  the  molecule  is  the  same  in 
both  varieties.  According  to  Prandtl  ^  its  empirical  formula  is 
HeMn5Al5(As,V)Si5028,  which  can  be  written  structurally  as  follows: 

/SiO^^MnCAlO^H^) 
Al— Si04=MnH 

^i04=Al— As04=Mn 
Al— Si04=MnH 

\siO,=Mn(A102H2) 

The  vanadic  ardennite  is  strictly  isomorphous  with  the  arsenical 
compound  and  in  it  V  replaces  As. 

Among  the  silicates  of  aluminum,  salts  of  orthodisilicic  acid  are 
very  rare.  The  only  one  which  seems  to  be  thoroughly  defined  is 
iolite,  which  agrees  best  with  the  formula 

^i^O^-AlMg 
Al— SiA-AlMg 

^iA.(A10H)2 
Al— SiA-AlMg 

^Si^O^-AlFe 

which  requires  the  following  percentage  composition: 

SiOg 49. 26 

AI2O3 33.50 

MgO 9.85 

FeO 5.91 

H2O 1.48 

100.00 

in  close  concordance  with  the  best  recorded  analyses.  By  alteration 
iolite  passes  into  mica,  going  through  an  intermediate  stage,  however, 
known  as  chlorophyllite.  This  substance  may  be  regarded  as  formed 
by  hydration,  in  which  the  linking  group  of  SizOy  in  iolite  is  split  into 
two  orthosilicic  radicles,  yielding  two  molecules  of  the  type 

Al— SiAm^Mg 
\si2O  =AlMg 

from  which  the  final  transition  into  a  mica  is  easy.  If  we  take  Ram- 
melsberg's  analysis  of  chlorophyllite,  recalculate  the  ferric  oxide  into 

1  Zeitschr.  Kryst.  Min.,  vol.  40,  p.  392, 1905. 


80 


THE   CONSTITUTION   OF   THE   NATURAL  SILICATES. 


alumina  and  lime  into  magnesia,  reducing  afterward  to  100  per  cent, 
we  get  the  following  comparison  between  observed  fact  and  the  com- 
position of  chlorophyllite  computed  from  the  foregoing  formula: 


Found. 


Reduced. 


Calculated. 


AI2O, 
Fe^O 
MgO. 
CaO. 
H2O. 


46.31 
25.17 
10.99 
10.91 
.58 
6.70 


100.  66 


47.99 
33.34 

11.74 
6.93 


100.  00 


48.39 
32.90 

12.90 
5.81 


100.00 


The  agreement  is  as  close  as  could  be  reasonably  expected.  The 
replacement  of  a  little  magnesia  by  a  little  water  in  the  original  altera- 
tion product  accounts  for  the  discrepancies. 

Two  other  magnesian  alumosilicates,  of  rather  uncertain  character, 
may  possibly  be  related  to  iolite;  namely,  the  lasallite  of  G.  FriedeP 
and  the  pilolite  of  Heddle.^  Both  minerals  are  highly  hydrous  and 
are  fairly  represented  by  the  subjoined  formulae: 


Lasallite. 
Al— SiA-Hs 

Al— SiaO^.Hs 
^Si^O^-AlH^ 


+  7H,0 


PHolite. 
^Si^O.-MgHe 

Al— SiA-Hs 

^iA.Mg2 

Al— SiA-Hs 


+7H20 


\ 


Si,0,.MgH, 


The  exact  hydration  is  doubtful  but  not  far  from  that  shown  in  the 
formulae.  There  are  a  number  of  other  questionable  minerals 
recorded,^  of  generally  similar  composition,  but  their  consideration 
in  detail  would  hardly  be  profitable.  Some  of  them  are  probably 
mixtures  of  magnesian  silicates  with  clays. 

Two  more  aluminous  minerals  are  probably  to  be  classed  as  ortho- 
disilicates.     One  barylite,  is  near 

ySijO^.AlBa 

Al— Si207.AlBa 

\si3O7.AlBa 

1  Soc,  min.  Bull.,  vol.  24,  p.  12, 1901;  idem,  vol.  30, p.  80, 1907. 

2  Min.  Mag.,  vol.  2,  p.  206, 1878. 

8  See  Dana's  System  of  mineralogy,  6th  ed.,  pp.  705-711, 1892. 


THE   SILICATES  OF  ALUMINUM.  '       g^ 

which  requires  the  following  percentage  composition: 

SiOa 

Al^O, 35.19 

BaO. 19.94 

44.87 


100.00 
The  other  silicate,  sphenoclase,  is  approximately  A\,(Sifi,)  fi^^,  which 
may  be  analogous  to  barylite  in  structure  or  written  as  a  calcium 
salt  similar  m  type  to  okenite.     Both  barylite  and  sphenockse,  how- 
ever, are  uncertain,  and  their  relations  are  not  definitely  known 

Well-defined   metasihcates   of   aluminum,   or   alumometasiHcates 
seem  to  be  few  in  number.     It  has  already  been  shown  that  many  of 
the  species  classed  as  metasihcates  are  really  mixed  salts,  trisihcates 
and  orthosiUcates  being  commingled. 

With  beryl,  however,  GlgAl^SieO^g,  the  evidence  in  favor  of  a  meta- 
siHcate  structure  is  fairly  good,  although  the  composition  can  also 
be  expressed  as  that  of  a  basic  trisiUcate.  There  are  thus  two 
alternatives, 


Ai4i03^^^ 

\sio. 


Gl 


Gl  and         Gl 


C> 


But  beryl  alters  into  mica,  a  fact  which  is  favorable  to  the  first  of 
these  formulae,  and  all  of  its  commoner  alterations  seem  to  take  place 
by  replacement  of  glucinum.  In  the  trisiUcate  formula  the  alumina 
should  be  equally  replaceable,  and  so  far  the  evidence  is  adverse  to  it. 
Furthermore,  Traube  ^  has  effected  the  synthesis  of  beryl  by  precipi- 
tating a  mixture  of  glucinum  and  aluminum  sulphates  with  a  solution 
of  sodium  metasiHcate,  and  then  crystalUzing  by  fusion  of  the  precipi- 
tate with  boron  trioxide.  Since  the  starting  point  was  a  metasilicate, 
there  is  a  fair  presumption  that  the  product  was  a  metasilicate  also. 
Beryl  can  be  written  as  a  pseudometasihcate,  but  there  are  no  data  to 
justify  doing  so. 

The  mineral  astroUte,  described  by  Eeinisch  ^  may  perhaps  be  a 
metasiHcate.  If  so  its  formula  is  (Na,K)Fe''(AlFe"0  (Si03)5.H20,  but 
the  species  needs  further  study.  Spodumene  and  jadeito  also  have 
apparently  metasiHcate  ratios,  but  on  mineralogic  grounds  it  is  best 

1  Neues  Jahrb,,  1884,  vol.  1,  p.  275.  *  Centralbl.  Mineralogie,  1904,  p.  108. 

43633°— Bull.  588—14 6 


82  THE   CONSTITUTION   OF   THE  NATUKAL  SILICATES. 

to  defer  their  consideration  and  to  take  them  up  in  connection  with 
the  pyroxenes  and  amphiboles. 

The  alumosiUcates  derived  from  metadisihcic  acid,  HgSigOs,  are 
few  in  number.  Two  of  them,  ptiloHte  and  mordenite,  have  already 
been  discussed  as  zeolites;  two  others,  petaUte  and  milarite,  demand 
attention  now.  The  formula  of  petaUte,  empirically,  is  AlLi  (81205)2, 
similar  to  the  metasilicate  formula  for  spodumene,  and  the  two 
species  are  commonly  associated.  Petalite,  however,  has  far  the 
lower  density  of  the  two  and  is  therefore  presumably  composed  of 
smaller  molecules.  An  alteration  product  of  petaHte,  hydrocastorite, 
approximates  roughly 

Al<f 

\Si2O  — H 

which  requires  the  following  percentage  composition : 

SiOa 60.  60 

AI2O3 25.  75 

H2O 13.  65 

100.  00 

The  actual  hydrocastorite  contains  about  4.3  per  cent  of  Hme,  and 
is  doubtless  impure.  By  Doelter  ^  petaUte  is  interpreted  somewhat 
differently,  being  given  the  empirical  formula  Al2Li2Siio024,  but  the 
formula  here  adopted  is  the  one  most  generally  received. 

Milarite,  HKCa2Al2(Si205)e,  is,  Hke  beryl,  hexagonal,  and  its  formula 
is  analogously  to  be  written 


— S120. 

\siA 


'5 


/SI2O5 
I— Si205- 

\si,0.— K 


Al— Si^Os— H 


For  this  species  the  only  evidence  is  that  of  its  composition.     Its 
genesis  and  its  possible  alterations  are  unknown. 

The  clays  are  peculiarly  difficult  to  interpret  constitutionally. 
One  jnember  of  the  group,  kaolin,  has  already  been  considered,  and 
this  member  is  peculiar  in  being  crystalline.  The  other  clays  are 
amorphous  and  of  uncertain  origin;  they  often  occur  in  complicated 
mixtures,  are  difficult  to  identify  with  certainty,  and  still  more  difficult 

1  Mln.  pet.  Mitt.,  1878,  p.  529. 


THE   SILICATES   OF  ALUMINUM.  33 

to  correlate  with  other  species.  They  undoubtedly  represent  the 
breaking  down  of  crystaUine  sihcates,  to  which  they  are  related 
somewhat  as  kaohn  is  related  to  the  feldspars,  but  rarely  if  ever  has 
their  actual  genesis  been  observed.  Furthermore,  the  'integrity  of 
some  of  the  clay  sihcates  is  in  question.  According  to  Stremme ' 
several  of  them  are  merely  mixtures  of  colloidal  alumina  and  colloidal 
siHca,  whereas  Thugutt '  regards  them  as  definite  compounds.  The 
controversy  can  probably  be  settled  only  by  careful  thermal  investi- 
gation as  to  the  character  of  the  hydration,  a  Hue  of  attack  which  has 
been  followed  to  some  extent  by  Le  ChateHer,^  although  not  yet  to  a 
finahty.  The  formulae  proposed  here  are  therefore  to  be  regarded  as 
merely  tentative  and  as  a  first  step  toward  the  better  study  of  the 
several  species. 

Upon  comparing  the  formula  of  aluminum  orthosilicate  with  that 
of  kaoUn  an  indication  of  serial  arrangement  becomes  evident,  which 
may  be  written  thus: 

Normal  salt.  Westanite  or  woerthite. 

^i04=Al  OH 

Al— SiO,^Al  Al— Si04=Al 

\si04=Al  \siO,=Al 

Kaolin.     ■  MontTnorillonite.  Newtonite 

/OH  .OH  ,0H 

Al— Si0,=H3  Al—SiO,=H3  Al— OH 

\SiO,=Al  \siO,=H3  \siO,=H3 

Woerthite  is  an  altered  siUimanite,  and  westanite  is  perhaps  a 
similar  derivative  of  andalusite.  The  newtonite  compound  has  al- 
ready appeared  in  the  mica  series  among  the  components  of  cookeite 
and  rumpfite. 

The  best  that  can  be  said  for  these  formulae  is  that  they  are  sug- 
gestive. In  one  respect  they  are  highly  questionable,  for  the  reason 
that  the  group  — SiO^^Hg  is  indicative  of  loosely  combined  water, 
whereas  in  these  particular  clays  the  water  is  quite  firmly  retained. 
On  this  point  much  fuller  information  is  needed,  and  future  evidence 
may  prove  that  the  serial  relation  indicated  is  apparent  only. 

The  composition  of  rectorite  may  be  Al3(Si04)3H3  4-  2H2O,  or  that  of 
a  hydrous  kryptotile.  Halloysite  has  the  composition  of  kaolin 
plus  one  molecule  of  water,  the  latter  being  removable  at  or  about 
100°.  Halloysite,  however,  differs  from  kaolin  in  being  decomposable 
by  hydrochloric  acid,  and  hence  it  is  unlikely  that  the  two  species 

1  Centralbl.  Mineralogie,  1911,  p.  205.  »  Zeitschr.  physikal.  Chemie,  vol.  1,  p.  396, 1887. 

2  Idem,  1911,  p.  97;  1912,  p.  35. 


84  THE    CONSTITUTION    OF    THE    NATURAL   SILICATES. 

have  similar  structure.  Allophane  is  perhaps  (A102H2)2H2Si04  +  SHjO 
or  it  may  be  written  analogously  to  andalusite, 

Al3(SiO,)3(A102H2)3.12H20. 

Neither  formula  is  sustained  by  any  good  evidence.  Other  clays 
are  possibly  as  follows: 

Samoite Al4(SiO4)3.10H2O 

Cimolite Al4(Si308)3.6H20 

Collyrite (A10)4Si04.6H20 

Schrotterite (A102H2)4Si04.6H20 

Melite (A102H2)4Si04.4H20 

One  other  member  of  this  group,  termierite,  has  the  empirical  for- 
mula Al2Si60i5.18H20.  It  may  be  a  metadisilicate,  but  until  the 
exact  character  of  its  hydration  is  determined  its  constitution  must 
remain  in  doubt. 

None  of  these  formulae  can  be  construed  as  anything  more  than  a 
temporary  suggestion,  which  may  help  research.  The  hydrous  ferric 
silicates  are,  if  anything,  less  satisfactory  than  the  aluminum  salts. 
Anthosiderite  is  representable  by  the  formula  Fe4(Si308)3.2H20,  and 
chloropal  by  the  expression  Fe2(Si04)3H6.  Nontronite,  according  to 
Weinschenk,^  is  the  ferric  equivalent  of  kaolin,  H4Fe2Si209,  but 
Bergeat,^  who  has  studied  the  mineral  as  a  derivative  of  kaolinite, 
assigns  it  the  more  complex  formula  H8Fe4Sii5028.  Miillerite  is  prob- 
ably a  metasilicate,  Fe2(SiOo)3.2H20.  Hisingerite  seems  to  range 
from  a  ferric  kaolin  to  a  ferric  halloysite,  and  further  than  this  it  is 
not  worth  while  to  go.  The  remaining  iron  clays  which  have  received 
specific  names  are  altogether  doubtful.  The  chromium  clays,  wol- 
chonskoite,  alexandrolite,  and  others,  are  also  of  very  uncertain 
character. 

Several  sihcates  of  the  rare  earths  may  be  properly  mentioned 
here  on  account  of  their  analogy  to  the  alumosilicates. 

To  cerite,  which  is  of  doubtful  composition,  the  provisional 
formula 

.Si04=CeO.H2 

Ce— Si04=CeO.H2 

\si04=CeO.H2 

may  be  assigned.  Other  earth  metals — lanthanum,  the  two  didy- 
miums,  and  others — replace  a  considerable  part  of  the  cerium. 
With  cerium  only  the  formula  requires  the  following  percentage 
composition : 

SiO. 20.22 

CeaOg 73.  71 

H2O 6.07 

100.  00 

1  Zeitschr.  Kryst.  Min,,  vol.  28,  p.  150, 1897.  » Centralbl,  Mineralogie,  1909,  p.  161. 


THE    SILICATES   OF   ALUMINUM.  85 

Beckelite  is  perhaps  similar  to  cerite  in  structure,  having  tlie 
probable  formula 

/Si04=CeO.Ca 

Ce— Si04=CeO.Ca 

\si04=CeO.Ca 

with  lanthanum,  neodymium,  praseodymium,  and  other  elements 
partly  replacing  cerium. 

The  yttrium  silicate,  thalenite,  is  possibly  represented  by  the  simple 
formula  Y^SiO^— R',  in  which  R'js  partly  H  and  partly  Na  or  K. 
The  ideally  pure  mineral,  however,  is  yet  to  be  found.  Another 
silicate  of  yttrium  and  calcium,  hellandite,  is  of  quite  different  type, 
and  agrees  well  with  the  formula 

/SiO,=(YO)3 
Ca<r 

\SiO,=H3 

Another  yttrium  mineral,  cenosite  or  kainosite,  appears  to  be  an 
orthodisilicate  containing  a  carbonic  radicle,  thus: 

/CO3-H 
Y— Si^O^.CaHe 
\si2O7.CaY 

The  recently  described  thortveitite  ^  is  much  simpler,  being  the 
normal  orthodisilicate  of  scandium, 

Sc^Si^Oy 

Britholite  and  erikite  are  phosphatosilicates,  but  the  published 
analyses  correspond  to  no  simple  formulae.  All  these  species  need 
fuller  investigation. 

Several  salts  of  triad  bases  may  be  noted  here  as  having  more 
analogy  to  the  compounds  of  aluminum  than  to  any  other  silicates. 
MelanotekJte,  for  instance,  is  a  silicate  of  lead  and  ferric  iron,  which, 
according  to  Warren,^  has  the  empirical  formula  Pb4Fe3Si30i5.  Struc- 
turally this  becomes 

.Si04=(FeO)Pb 

Fe— SiO=(FeO)Pb 

\siO,=(FeO)Pb 

Kentrolite  is  probably  similar,  but  with  Mn'"  replacing  Fe'". 


See  Schertelig,  Centralbl.  Mineralogie,  IQU,  p.  721.      »  Am.  Jour.  Sci.,  4th  ser.,  vol.  0,  p.  116, 1898 . 


86  THE   CONSTITUTION   OF   THE   NATURAL  SILICATES. 

Glauconite,  FeK(Si03)2.^H20,  is  a  hydrous  silicate  which  is  rarely 
found  in  even  approximate  purity.  The  formula,  therefore,  merely 
represents  the  best  evidence  we  have  as  to  the  constitution  of  the 
mineral.  It  may  be  a  pseudometasilicate  and  equivalent  to  a  hydrated 
potassium  acmite,  a  supposition  which  would  seem  to  be  capable  of 
experimental  verification.  Some  iron  is  commonly  replaced  by 
aluminum  and  some  potassium  by  other  basic  radicles.  Glauconite 
is  of  marine  origin,  but  celadonite,  formed  by  alteration  in  certain 
volcanic  rocks,  is  probably  the  same  compound. 

Pseudobrookite  is  an  orthotitanate  of  iron,  Fe4(Ti04)3,  and  arizonite 
is  the  metatitanate,  Fe2(Ti03)3.  There  are  also  the  bismuth  silicates, 
eulytite  and  agricolite,  Bi^ (8304)3,  which  differ  in  form  but  are 
identical  in  empirical  composition.  They  therefore  suggest  two  types 
of  chemical  structure  among  the  silicates  of  trivalent  bases. 


CHAPTER  IV. 
SILICATES   OF  DYAD  BASES. 

OBTHO  SILICATE  S . 

Although  the  orthosilicates  of  the  dyad  metals  are  presumably  sim- 
pler than  those  of  aluminum,  the  problem  of  their  constitution,  studied 
in  the  Hght  of  mineralogic  evidence,  is  pecuHarly  difficult.  Starting 
points  exist,  in  the  salts  of  magnesium,  iron,  manganese,  zinc,  and 
glucinum,  but  the  derivatives  are  fewer  than  in  the  case  of  aluminum, 
and  the  evidence  upon  which  to  base  argument  is  correspondingly 
Umited. 

Expressed  in  the  simplest  terms,  the  normal  orthosihcates  of  this 
group  are  represented  by  the  general  formula  R2Si04.  To  this  type 
the  following  minerals  correspond: 

Forsterite Mg2Si04 

Fayalite Fe2Si04 

Tephroite Mn2Si04 

Willemite Zn2Si04 

Phenakite Gl2Si04 

Monticellite CaMgSi04 

Knebelite MnFeSi04 

Glaucochroite CaMnSi04 

Between  these  rninerals  there  are  many  intermediate  species  or 
varieties,  which  may  be  either  isomorphous  naixtures  or  double  salts 
representing  polymers  of  the  fundamental  type.  Thus,  chrysolite  or 
oHvine  ^  may  be  a  mixture  of  forsterite  and  fayahte,  or,  in  the  case  of 
hyalosiderite,  a  salt  of  the  formula  Mg4Fe2(Si04)3.  So  also,  allied  to 
knebehte,  we  have  igelstromite,  Fe4Mn2(Si04)3,  and  in  trimerite  we 
find  the  salt  Gl3Mn2Ca(Si04)3. 

A  close  study  of  the  derivatives  of  these  normal  salts  shows  that  the 
assumption  of  polymerization  seems  to  be  necessary.  If  the  theory 
of  substitution  is  vaHd,  then  the  existence  of  polymers  must  be  taken 
for  granted,  and  it  thus  becomes  possible  to  develop  a  system  of 
formulae  which  satisfies  all  the  conditions  imposed  by  the  evidence 
now  at  hand.  For  some  of  the  species  already  mentioned  the  degree 
of  polymerization  is  difficult  to  determine,  and  synthetic  investiga- 
tions seem  to  be  needed.  In  other  cases  the  problem  is  comparatively 
simple,  an*  the  indications  as  to  the  true  formulae  are  apparently  clear. 

1  Tschermak  regards  olivine  as  a  basic  metasilicate,  but  that  interpretation  is  difficult  to  reconcile  with 
an  the  evidence.  «- 


88 


THE   CONSTITUTION   OF   THE    NATURAL   SILICATES. 


For  instance,  a  good  example  is  furnished  by  the  chondrodite  group, 
for  which  we  have  the  empirical  formulae  estabhshed  by  Penfield  and 
Howe.^     Structurally  written  these  become 


Clinohumitc. 
Mg 

A 


Humite. 

Mg 


Chondrodite. 

I 

(MgF), 


(MgF): 


or  derivatives  respectively  of  the  salts  Mg8(Si04)4,  Mg6(Si04)3,  and 
Mg4(Si04)2,  with  one  atom  of  magnesium  in  each  case  replaced  by  the 
two  univalent  — Mg — F  groups.  Prolectite  is  a  fourth  member  of 
the  series,  described  by  Sjogren, ^  which,  on  crystallographic  grounds, 
is  supposed  to  be  simply  Mg.=Si04=(MgF)2,  although  it  has  not  been 
analyzed.  The  fluorine  in  these  minerals  is  usually  replaced  in  part 
by  hydroxyl,  but  the  replacement  is  rarely  complete.  The  mineral 
leucophoenicite  ^  is  a  similar  compound,  with  a  formula  analogous  to 
that  of  humite,  namely,  Mn5(Si04)3(MnOH)2,  but  containing  slight 
replacements  of  manganese  by  calcium  and  zinc.  Gageite  is  also  a 
member  of  the  humite  group,  and  its  formula  can  be  written 


(KOH). 


in  which  R  represents  Zn,  Mg,  Mn,  in  the  ratio  1:3:7.     It  is  closely 
related  to  leucophoenicite.* 

Clinohedrite  occurs  in  association  with  forsterite,  and  the  two 
species  have  nearly  the  same  specific  gravity.  Hence  forsterite  may 
be  Mgg  (8104)4,  and  this  is  the  only  datum  available  from  which  to 
infer  its  molecular  magnitude.  The  synthetic  transformation  of 
forsterite  into  clinohumite,  if  it  could  be  effected,  would  go^f ar  toward 
setthng  the  question. 

1  Am.  Jour.  Sci.,  3d  ser.,  vol.  47,  p.  188, 1894. 

2  Geol.  Inst.  Upsala  Bull.,  vol.  1,  p.  40,  1892-93;  idem,  vol.  2,  p.  99,  1894-95. 

3  See  Penfield  and  Howe,  Am.  Jour.  Sci.,  4tli  ser.,  vol.  8,  p.  351, 1899. 
<  Phillips,  Am.  Jour.  Sci.,  4th  ser.,  vol.  30,  p.  283, 1910. 


SILICATES  OF   DYAD  BASES.  gQ 

In  the  case  of  phenakite  the  triple  formula  01^  (8104)3  is  rendered 
probable  b}^  the  existence  of  trimerite,  Gl3Mn2Ca(Si04)3.  It  is  also 
emphasized*  by  the  species  helvite  and  danaUte,  which  contain 
sulphur,  probably  combined  in  the  dyad  group  — R— S— R— . 
Both  these  species,  in  all  their  known  occurrences,  agree  with  the 
general  formula 

Gl 


L 


in  which  R  may  be  either  Fe'',  Mn,  or  Zn.  The  R  is  variable,  but 
the  other  constituents  are  constant.  In  helvite,  manganese  and 
iron  occur,  and  in  danahte  zinc  appears.  In  the  Colorado  danalite 
zinc  predominates  largely  over  iron,  and  there  is  very  Httle  manga- 
nese. The  Rockport  danahte  has  iron  in  excess  of  zinc,  and  rather 
more  manganese.  The  Cornish  danahte  is  very  low  in  zinc,  and  the 
iron  largely  exceeds  the  manganese.  The  ratio  Gl:Si04::3:3,  how- 
ever, holds  for  all. 

If  phenakite  is  Gl8(Si04)3,  then  willemite,  which  is  morphologically 
similar,  is  probably  Zne(Si04)3,  with  zinc  partly  replaced  by  manga- 
nese in  the  variety  known  as  troostite. 

Spurrite,^  empirically  2Ca2Si04.CaC03,  may  possibly  be  assigned 
a  structure  similar  in  type  to  those  used  in  the  preceding  groups  of 
minerals,  thus: 

Ca 

/Si04\ 

Ca<  >Ca 

^8104^ 

Ca— CO3— Ca 

but  with  no  evidence  to  go  upon  other  than  its  composition.  To  the 
associated  hillebrandite  the  formula  Ca2Si04.H20  has  been  given, 
but  the  water  in  it  may  be  constitutional.  In  that  case  hillebrandite 
becomes  a  basic  metasihcate,  (CaOH)2Si03.  Molybdophylhte, 
PbMgSi04.H20,  may  be  similarly  constituted. 

The  three  orthorhombic  species,  bertrandite,  calamine,  and  ilvaite, 
are  most  conveniently  represented  as  derivatives  of  the  bipolymer 
R4(Si04)2,  hke  chondrodite.     Crystallographically  ilvaite  resembles 

1  See  Wright,  F.  E.,  Am.  Jour.  Scl.,  4th  ser.,  vol.  26,  p.  647, 1908. 


90 


THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 


humite,  but  bertrandite  and  calainine  are  related  to  each  other. 
The  simplest  analogous  formulae  for  the  three  minerals  are  as  follows : 


Bertrandite 

Gl 


ySiO,. 
Gl<  >G1 

H^         X^l— OH 


Calamine. 

Zn<  >Zn 

II 
(ZnOH), 


Ilvaite. 
Ca 

Al— OH 


Calamine  may  also  be  written  as  a  metasilicate,  (ZnOH)2Si03,  with 
half  of  the  formula  indicated  above,  but  then  the  analogy  with  ber- 
trandite disappears.  The  structure  proposed  is  therefore  prefer- 
able, at  least  until  more  evidence  has  been  accumulated.  CHnohe- 
drite  is  equivalent  to  calamine,  but  with  half  the  zinc  replaced  by 
calcium.  Zamhonini  ^  interprets  calamine  as  a  basic  orthodisiHcate. 
Baschieri^  regards  ilvaite  also  as  an  orthodisilicate.  It  is  not  easy 
to  write  constitutional  formulae  for  these  minerals  and  to  show  their 
relation  to  other  species  on  that  basis.  Further  investigation  is 
evidently  needed. 

To  the  datolite  group  a  similar  constitution  is  ascribable.  The 
species,  reduced  to  their  simplest  empirical  expressions,  are  these: 

Datolite HCaBSiOg 

Homilite CasFeBaSigOio 

Euclase HGlAlSiOg 

Gadolinite GlgFeYgSiaOio 

By  doubUng  the  formulae  of  datoHte  and  euclase  all  four  of  the 
minerals  become  similar  in  constitution.  Hydrogen  here  is  evi- 
dently basic,  and  boron  must  play  the  same  part  as  aluminum  and 
yttrium.  Assuming  these  elements  to  be  present  in  the  univalent 
groups  BO,  AlO,  and  YO,  the  subjoined  formulae  follow: 

Datolite.  Euclase. 

Hg  H2 

(BO),  (AlO), 

1  Contributo  alio  studio  dei  silicati  idrati,  1908.         2  Zeitschr.  Kryst.  Min.,  vol.  49,  p.  112, 1911 . 


SILICATES  OF  DYAD  BASES.  91 

Eomilite.  Gadolinite. 

Fe  Fe 

(BO),  (yI)), 

Gadolinite  alters  with  great  ease,  passing  by  hydration  into  a 
brownish-red  earthy  substance.  The  analyses  of  this  substance, 
whicli  is  probably  never  pure  and  definite,  are  not  altogether  satis- 
factory, but  they  indicate  in  a  general  way  a  transformation  into  the 
compound 

Fe'^OH 

<:o;> 

II 

(YO,H,), 

to  which  the  alteration  product  very  roughly  approximates. 

It  will  be  observed  that  all  of  the  foregoing  structural  formulae  in 
this  group  of  compounds  are  rings  or  series  of  rings.  From  them, 
however,  chainlike  molecules  are  derivable,  and  these  seem  to  exist 
in  friedelite,  pyrosmalite,  and  dioptase.  The  last-named  mineral, 
simply  written,  is  CuH3Si04,  but  it  is  morphologically  related  to  the 
two  other  species,  which  have  a  much  greater  complexity  of  composi- 
tion. The  following  expressions  derived  from  the  polymer  RgCSiO^)^ 
are  probably  the  best  to  represent  existing  evidence : 

Friedelite.  Dioptase. 

/Si04=H2(MnCl)  ySiO^^Ha 

Mn<  Cu< 

\SiO,=H2  >SiO,=H2 

Mn<  Cu< 

\Si04=H2  ^SiO^^H^ 

Mn<  Cu< 

\SiO,=HMn  \Si04=HCu 

Pyrosmalite  is  like  friedelite,  but  a  large  part  of  the  manganese  is 
replaced  by  iron.  Possibly  karyopilite  may  be  similar,  having  the 
formula 

/SiO,=H2(MnOH) 
Mn< 

NSiO^^H^ 
Mn< 

\Si04=HMn 


92  THE   CONSTITUTION    OF   THE    NATURAL   SILICATES. 

These  formulae  are  purely  tentative  and  need  additional  support. 
By  synthetic  and  genetic  investigations  they  may  be  supported  or 
overthrown.  That  they  sustain  one  another  and  fit  in  well  with  the 
formulae  of  the  preceding  species  is  all  that  can  be  said  in  their  favor. 
Palache  ^  and  Zambonini  ^  assign  more  complex  formulae  to  friedelite. 
The  chlorine  in  that  mineral  seems  to  be  partly  replaced  by  hydroxyl. 

Bementite  is  still  another  silicate  of  similar  t3^e.  In  accordance 
with  the  latest  analysis  by  Steiger,^  its  formula  is  simply  written: 

/Si04=MnH 
Mn< 

>SiO,=H2 
Mn< 

>iO,==H3 
Mn< 

\SiO,=MnH 

For  serpentine,  H4Mg3Si209,  several  formulae  are  possible,  and  con- 
cerning them  there  has  been  much  discussion.  The  species  commonly 
originates  in  nature  from  the  alteration  of  olivine  .on  the  one  hand  and 
from  pyroxene  or  amphibole  on  the  other,  and  it  is  therefore  conceiv- 
able that  it  may  include  two  or  more  isomeric  compounds.  In  favor 
of  this  supposition  there  is  some  evidence  but  nothing  conclusive. 
Massive  serpentine,  chrysotile,  antigorite,  and  other  allied  varieties 
differ  in  their  physical  properties  and  suggest  the  existence  of  isomers, 
but  much  more  investigation  is  needed  in  order  to  decide  whether  the 
supposition  is  true  or  not. 

By  some  authorities  serpentine  is  regarded  as  an  orthosilicate  and 
by  others  as  a  salt  of  the  acid  HeSigOy.  On  the  latter  supposition  it 
becomes  Mg=Si207^H2(MgOH)2,  which  may  be  derived  either  from 
2Mg2Si04  or  2MgSi03,  with  loss  of  magnesium  in  one  case  and  gain  in 
the  other.  On  the  orthosilicate  basis  it  is  simply  derivable  from  the 
polymer  Mg4 (8104)2  and  is  related  to  the  intermediate  alteration 
product,  villarsite,  as  follows: 

Mg^{SiO^)2-  Villarsite.  Serpentine. 

Mg  Mg  H, 

II  II  .      If 

.Si04V  .Si04.  .Si04V 

Mg<;         >Mg         Mg<         >Mg  Mg<;         >Mg 

\SiO/  ^8104/  \Si04^ 

Mg  H  Mg— OH         H  Mg— OH 


1  Am.  Jour.  Sci.,  4th  ser.,  vol.  29,  p.  177, 1910.  »  See  Palache,  idem,  p.  182. 

2  Contributo  alio  studio  dei  silicati  idrati,  1908. 


SILICATES  OF   DYAD  BASES.  93 

On  this  scheme  the  formula  for  serpentine  corresponds  with  that  of 
chrondrodite;  and  the  fact  that  the  latter  mineral  alters  readily  into 
serpentine  is  strong  evidence  in  its  favor.  In  short,  that  formula 
best  indicates  the  genetic  relationships  of  serpentine,  and  on  such 
grounds  is  preferable  to  the  alternative  diorthosihcate  expression. 
The  latter  is  not  disproved;  it  is  simply  rendered  less  advantageous 
as  regards  existing  evidence,  which  is  the  evidence  now  to  be  inter- 
preted. 

In  some  former  investigations,  carried  on  jointly  with  Dr.  Schnei- 
der,i  I  sought  to  obtain  experimental  data  in  support  of  the  ortho- 
siHcate  formula  here  assigned  to  serpentine.  By  acting  on  ser- 
pentine with  dry  gaseous  hydrochloric  acid  we  found  that  a  part  of 
the  magnesium  could  be  removed  as  chloride,  whereas  olivine  and 
the  magnesian  micas  were  not  attacked.  At  first  it  seemed  probable 
that  the  reaction  would  give  a  quantitative  measure  of  the  magne- 
sium combined  as  MgOH;  but  our  later  experiments  and  those  of 
Lindner  ^  have  shown  that  the  expectation  was  not  well  founded. 
I  stiU  beheve,  however,  that  the  reaction  discriminates  between  those 
magnesium  siUcates  which  contain  MgOH  and  those  which  contain 
Mg  and  H  combined  othenvise,  for  only  the  members  which  must 
belong  to  the  first  class  are  acted  upon  by  the  reagent.  Brauns's 
objections  ^  to  this  supposition,  on  the  ground  that  the  dry  hydro- 
chloric acid  becomes  moist,  are  not  weU  taken,  for  the  reaction 
always  took  place  at  temperatures  lower  than  those  at  which  water 
is  given  off.  His  criticisms  may  apply  to  the  later  stages  of  the 
reaction,  after  it  has  once  fairly  begun,  but  not  to  its  initiation.  The 
magnesian  micas  which  contain  several  per  cent  of  water  are  aU 
decomposable  by  aqueous  hydrochloric  acid,  but  are  scarcely  touched 
by  the  dry  gas;  whereas,  on  the  other  hand,  serpentine  and  the 
chlorites  are  strongly  attacked.  After  the  gaseous  acid  has  acted  it 
becomes  moist,  but  very  slowly,  and  most  of  the  moisture  is  carried 
past  the  mineral  under  investigation  before  it  has  had  time  to  pro- 
duce an  appreciable  effect.  It  is  possible,  however,  that  a  slow 
stream  of  the  acid  may  act  differently  from  a  rapid  current,  and  that 
the  discordant  results  of  observation  may  be  due  to  differences  of 
this  kind. 

When  serpentine  is  ignited  water  is  expelled,  and  a  residue  having 
the  composition  MggSigOy  is  left  behind.  According  to  Kammels- 
berg  ^  the  water  is  given  off  in  two  portions — one-half  upon  weak 

1  U.  S.  Geol.  Survey  Bull.  78,  p.  11, 1891;  Bull.  90,  p.  11, 1892;  and  Bull.  113,  pp.  27  and  34, 1883. 

2  A.  Lindner,  Inaugural  Dissertation,  Breslau,  1893. 
•'  Neues  Jahrb.,  1894,  vol.  1,  p.  205. 

«  Handbuch  der  Mineralchemie,  2d  ed.,  p.  506, 1875. 


94  THE   CONSTITUTION    OF   THE    NATURAL   SILICATES. 

ignition,  the  other  after  heating  more  strongly.  On  the  orthosili- 
cate  theory  these  stages  may  be  represented  thus : 

Serpentine.  First  stage.  Second  stage. 

H^         \Mg— OH  ^^ 

At  the  end  of  the  second  stage,  if  the  ignition  has  not  been  too 
intense,  the  residue  is  still  decomposable  by  hydrochloric  acid,  but 
by  prolonged  heating  it  is  broken  up  quantitatively  into  soluble 
olivine  and  insoluble  enstatite. 

Allied  to  serpentine  is  the  somewhat  doubtful  picrosmine,  to  which 
the  formula  Mg2H2Si207  is  commonly  assigned.  Although  this  expres- 
sion suggests  a  diortfhosilicate,  it  may  also  be  written 


/SiO.,- 
SiO,- 


M< 


Mg 


i 


which  represents  picrosmine  as  a  dehydrated  serpentine  altered  sub- 
sequently by  rehydration,  with  replacement  of  one  magnesium  atom 
by  two  of  hydrogen.  This  mode  of  interpretation  brings  the  mineral 
into  line  with  serpentine,  and  all  the  known  relations  of  the  species 
are  adequately  expressed. 

Several  other  hydrous  silicates  seem  to  belong  in  this  group,  but 
their  nature  is  altogether  doubtful.     Thus  we  have 

Aphrodite Mg2H4(Si04)2 

Kerolite Mg(SiO,)2H5(MgOH) 

Nepouite,  H4Ni3Si209,  seems  to  be  the  nickel  equivalent  of  ser- 
pentine. Garnierite  and  noumeite,  other  Jiydrous  silicates  of  nickel, 
are  too  variable  in  composition  to  be  definitely  classed. 

METASILICATES. 

Although  the  metasilicates  appear  at  first  sight  to  be  extremely 
simple,  they  are  actually  rather  difficult  to  interpret.  It  is  easy 
enough  to  deduce  their  empirical  formulae  and  to  write  them  after- 
wards in  structural  terms,  but  this  is  not  sufficient.  The  structural 
formulae  must  express  all  known  relations  for  each  species,  and  in 
attempting  to  satisfy  the  established  conditions  the  difficulties  begin 
to  appear.     In  the  first  place,  metasilicic  acid  itself  is  defectively 


SILICATES  OF   DYAD  BASES.  95 

known,  and  no  ester  of  the  form  RjSiOg  h^s  yet  been  certainly 
obtained.  Troost  and  Hautefeuille's  ester  (CaHgJaSi^Oiz  suggests  the 
possibility  that  metasilicic  acid,  like  metaphosphoric  acid,  may  poly- 
merize, but  an  attempt  to  draw  general  conclusions  on  so  important 
a  question  from  one  datum  only  would  be  most  unwise.  The  possi- 
bility of  polymeric  acids,  however,  was  clearly  shown  in  the  general 
discussion  of  the  silicic  acids  in  Chapter  II  of  this  bulletin.  It  is  also 
emphasized  by  the  existence  of  four  distinct  modifications  of  mag- 
nesium metasilicate,  as  proved  by  Allen,  Wright,  and  Clement.* 
Whether  these  modifications  represent  different  metasilicic  acids  or 
not  is  yet  to  be  discovered. 

Again,  as  we  have  repeatedly  seen,  a  mineral  m,ay  be  apparently  a 
metasilicate  and  yet  really  a  mixture  of  orthosilicates  and  trisilicates. 
Even  a  basic  trisilicate  can  have  seemingly  metasilicate  ratios.  All 
these  considerations  complicate  the  identification  and  study  of  thp 
true  metasilicates  to  such  an  extent  that  only  provisional  conclusions 
can  be  drawn  from  the  data  now  on  hand. 

A  crystallized  silicate  of  sodium,  NagSiOa.SHgO,  is  well  known.  A 
solution  of  this  salt  added  to  a  solution  of  calcium  chloride  precipi- 
tates a  compound  which,  dried  over  sulphuric  acid,  has,  according  to 
my  own  observations,  the  composition  CajSigOg.SHgO.  This,  minus 
the  water,  is  analogous  to  the  mineral  wollastonite,  from  which  another 
mineral,  pectolite,  is  derived.  If  wollastonite,  instead  of  the  formula 
CajSijOe  be  given  the  formula  CagSigO^,  it  may  be  compared  structur- 
ally with  pectolite,  as  follows: 

Wollastonite. 

Ca<  \ 

NSiOg      >Ca 
Ca<  / 

It  must  be  remembered  that  the  molecular  weights  of  the  inorganic 
silicates  are  not  known  but  only  assumed;  and  the  problem  suggested 
by  the  foregoing  expressions  is  to  find  a  set  of  structural  formulae 
which  shall  represent  all  the  available  evidence.  Now,  wollastonite 
is  commonly  classed  with  the  pyroxenes,  on  crystallographic  grounds; 
and  so  too  is  pectolite.  But  both  species  are  very  easily  decomposed 
by  even  dilute  and  weak  acids,  whereas  the  normal  pyroxenes  are 
quite  refractory,  and,  furthermore,  wollastonite  has  a  lower  density 
than  any  pyroxene  which  approaches  it  in  composition.  Chemically, 
then,  these  species  are  dissimilar,  and  it  is  very  doubtful  whether  they 
can  properly  be  grouped  together.  That  wollastonite  and  pectolite 
are  true  metasilicates,  however,  is  sustained  by  the  fact  that  when 

1  Am.  Jour.  Sci.,  4th  ser.,  vol.  22,  p.  385, 1906. 


96  THE    CONSTITUTION    OF    THE    NATURAL   SILICATES. 

pectolite  is  ignited  one-sixth  of  the  silica,  proportional  to  the  hydrogen 
of  the  mineral,  is  split  off  in  the  free  state  and  can  be  determined  quan- 
titatively. That  is,  two  molecules  of  pectolite  are  decomposed  with 
liberation  of  metasilicic  acid,  HjSiOg,  which,  in  turn,  divides  into 
SiOz  +  HjO.  In  some  varieties  of  pectolite  manganese  replaces  part 
of  the  calcium,  and  schizolite  is  a  mineral  of  similar  composition  but 
with  different  ratios,  its  formula  approximating  H3Na3Ca3Mn2(Si03)8. 
Schizolite,  however,  may  be  an  isomorphous  mixture  of  two  silicates 
and  not  a  simple  compound.  It  is  in  no  true  sense  equivalent  to 
pectolite. 

Empirically  the  nonaluminous  pyroxenes  resemble  wollastonite  in 
their  ratios.  Thus  we  have,  according  to  the  commonly  accepted 
formulae  developed  by  Tschermak,  Doelter,  and  others : 

Enstatite,  orthorhombic MgaSigOg 

Diopside,  monoclinic CaMgSigOe 

Hedenbei^ite,  monoclinic CaFeSiaOg 

Rhodonite,  trlclinic MngSigOg 

There  is  also  a  great  number  of  other  intermediate  species  or  isomor- 
phous mixtures  in  the  pyroxene  series,  such  as  bronzite,  hypersthene, 
schefferite,  sahlite,  jeffersonite,  and  fowlerite,  in  which  we  find, 
variously  replacing  one  another,  salts  of  magnesium,  calcium,  iron^ 
manganese,  and  zinc.  All  these  minerals,  however,  conform  to  the 
general  formula  ESiOg,  or  112^1206,  which  adequately  expresses  their 
constitution  so  far  as  they  alone  are  concerned.  This  formula  can 
be  written  structurally — 


Si03 
SiO 


r/       >R 


> 


which  would  be  satisfactory  if  the  pyroxene  series  ended  here  and  if 
the  amphiboles  were  unknown. 

Going  a  step  further  we  find  in  augite  a  pyroxene  containing 
aluminum  and  having  an  oxygen  ratio  greater  than  in  the  group 
SiOg.  In  place  of  aluminum  ferric  iron  also  occurs,  and  alkalies  are 
sometimes  present.  Leaving  these  variations  out  of  account,  for 
consideration  later,  we  have  in  augite,  as  interpreted  by  Tschermak, 
together  with  the  normal  compound  112^1206,  the  basic  salt  RAlgSiOg. 
This  substance,  however,  is  not  known  by  itself,  unless  it  is  repre- 
sented by  kornerupine,  although  the  latest  analysis  of  that  mineral 
tends  to  negative  the  supposition.  An  artificial  silicate  of  similar 
type,  K2Al2Si06  has  been  prepared  by  Weyberg,^  and  may  serve  to 
strengthen  Tschermak' s  assumption.  This  salt  constitutionally  may 
be  regarded  as  a  basic  orthosilicate — 

1  Centralbl,  Mineralogie,  1911,  p.  326. 


SILICATES  OF   DYAD  BASES.  97 

/Al— 0~K 


SiO,< 

\.A1— O— K 

and  its  magnesian  equivalent  as 

^Al-0. 

but  neither  expression  is  to  be  taken  as  final.  The  excess  of  oxygen 
in  the  pyroxenes  may  be  due,  with  equal  probability,  to  the  equiva- 
lent group 


Al<  _  >Mg 


which  has  been  assumed  in  preceding  sections  of  this  memoir. 

Still  another  series  of  silicates  containing  triad  bases  and  also  alka- 
lies are  classed  with  the  pyroxenes,  as  follows : 

Spodumene AlLiSiaOg 

Jadeite AlNaSiaOg 

Acmite .Fe^^^NaSiaOg 

and  their  empirical  formulae   are  fairly  satisfactory.     Structurally 
these  expressions  become,  as  metasilicates, 

\SiO3— R' 
and  babingtonite,  which  contains  no  alkalies,  is  similar,  thus: 

^SiOg 


SiO, 


Fe"    +    3  R" 
/SiO, 


C>" 


Fe-Y 

^SiO 


E,''  being  =Ca,  Fe'',  and  Mn.  The  ferric  molecule  is  evidently 
equivalent  to  two' acmite  molecules,  with  Nag  replaced  by  a  linkmg 
atom  of  iron. 

So  far,  except  partially  in  the  comparison  between  wollastomte 
and  pectolite,  the  formulae  cited  for  the  pyroxenes  express  composi- 
tion  and   composition   only.     But   spodumene,    as   shown  by   the 
elaborate  research  of  Brush  and  Dana,  spHts  up  on  alteration  into  a 
43633°— Bull.  588—14 7 


98  THE   CONSTITUTION   OF   THE   NATURAL  SILICATES. 

mixture  of  eucryptite,  an  orthosalt,  and  albite,  a  trisilicate.  This 
observation  suggests  two  alternatives:  Either  that  spodumene  is 
derived  from  a  polymetasilicic  acid,  or  else  that  it  is  a  pseudometa- 
silicate,  a  mixed  ortho-  and  tri-salt,  like  some  of  the  species  which 
have  already  been  explained.  An  analogy  with  leucite,  for  example, 
will  at  once  be  inferred,  and  that  species,  empirically,  is  strikingly 
like  spodumene,  thus : 

Leucite AlKSiaOg 

Spodumene AlLiSi^Oo 

Like  spodumene,  leucite  alters  into  a  feldspar  and  a  member  of  the 
nepheline  group,  but  it  differs  from  spodumene  in  form  and  in 
density.  The  specific  gravity  of  the  isometric  leucite  is  2.5,  that  of 
the  monoclinic  spodumene  is  nearly  3.2,  and  hence  we  may  reasonably 
infer  that  spodumene  has  the  larger  and  more  condensed  molecule. 
In  order  to  explain  the  relations  of  leucite,  its  empirical  formula  was 
quadrupled,  and  in  that  way  a  relation  with  the  garnet  group  was 
brought  out.  For  spodumene,  regarding  it  also  as  a  mixed  siHcate, 
a  sixfold  multipUcation  of  its  formula  indicates  its  greater  density, 
and  its  splitting  up  into  eucryptite  and  albite,  with  partial  replace- 
ment of  lithium  by  sodium,  is  representable  as  follows: 

1^2  Li 

/SigOs— Al=Si04\ 

A;feSi308— Al<  >A1— Si04=Al 

XSigOs^Al— SiO/ 

I.  I! 

Li  Lig 

From  this  grouping  of  atoms  the  transition  into  Al3(Si308)3Na3  + 
AI3  (Si04)3Li3  is  hardly  more  than  simple  cleavage,  and  the  relations 
between  the  three  species  are  intelligibly  expressed.  Acmite,  which 
yields  pseudomorphs  of  analcite,  and  jadeite,  also,  probably  follow 
the  same  rule,  the  formula  of  one  being  typical  of  the  others.  The 
ferric  molecule  in  babingtonite  should  be  still  another  instance  of  the 
same  kind,  with  Fe''3  in  place  of  Lig,  and  Fe'^'  instead  of  Al.  Pos- 
sibly pyrophyUite,  HAlSijOg,  is  related  to  spodumene  and  jadeite, 
much  as  kaoHnite  is  related  to  the  feldspars.  PyrophyUite  is  prob- 
ably a  pseudometasilicate,  for  silica  is  not  liberated  from  it  upon 
ignition,  at  least  not  to  any  noteworthy  extent. 

If  the  formula  just  developed  for  spodumene  should  be  sustained, 
it  would  seem  necessary  to  adjust  the  other  pyroxenes  with  it. 
For  Tschermak's  aluminous  constituent  of  augite  this  adjustment  is 
easily  made  by  taking  the  formula  AlaMgSiOe  six  times,  as  was  done 


SILICATES   OF   DYAD  BASES.  99 

for  spodumene.  The  paralleUsm  between  the  two  species  is  then 
representable  as  follows : 

Spodumene. . .     Al,(Si30«)3(SiO,)3Li« 

Alummum-augite Alo(SiO,)3(SiO,)3(A102Mg), 

the  univalent  A102Mg  having  been  recognized  among  the  micas. 
This  formula  serves  to  explain  the  weU-known  alterabihty  of  augite 
into  epidote  and  into  mica,  and  so  far  at  least  is  useful.  I  do  not, 
however,  feel  incHned  to  put  very  much  stress  upon  it,  for  as  yet  it  i^ 
only  an  expression  of  analogy,  which  may  or  may  not  prove  to.be 
valid.  It  would  seem  to  require  the  recognition  of  all  the  pyroxenes 
as  pseudometasiiicates,  in  which  case  the  normal  series,  containmg 
only  dyad  bases,  would  become 

R  R 

II  II 

/SigOs— R— SiO,V 

R<  >R 

\Si3O— R— SiO/ 


I 


R 


where  four  atoms  of  R  are  given  linking  functions,  and  the  other 
four  are,  so  to  speak,  replaceably  combined.  On  this  basis  we 
should  write 

Diopside Mg4Ca4(Si308)2(Si04)2 

Hedenbergite Fe4Ca4(Si308)2(Si04)2 

The  formula  R "4 (AlOg (8104)2  (Si04)2  would  be  exactly  parallel  with 
these,  and  affords  another  expression  for  Tschermak's  compound, 
Al2RSi06.  Or  the  ground  of  simplicity  this  is  preferable  to  the  more 
complex  expression  based  on  the  formula  of  spodumene.  If  to  mon- 
ticelUte  we  assign  the  quadrupled  formula  Mg4Ca4  (8104)4,  and  to  for- 
sterite  the  similar  formula  Mgg (8104)4,  diopside  becomes  equivalent 
to  them  in  structure,  with  one-half  the  orthosilicic  radicle  replaced 
by  the  trisiUcic  SigOg.  The  pyroxenes  and  the  oUvines  thus  appear  to 
be  curiously  related  compounds,  although  they  are  unlike  morpho- 
logically. 

This  mode  of  interpreting  the  pyroxenes  is  so  remote  from  our 
usual  conceptions  that  I  offer  it  with  great  diffidence.  It  unifies  the 
group,  however;  it  expresses  the  observed  alterations  of  the  several 
species;  and  despite  its  complexity  it  will  be  found  to  be  sustained 
and  strengthened  by  evidence  brought  out  in  the  study  of  the 
amphiboles. 

This  last-named  group  of  highly  important  minerals  resembles  the 
P3rroxenes  in  composition  and  is  explained  by  Tschermak  in  essen- 
tially the  same  way.     Their  mJleiaii^'  weights,  ^howeyei-,  are  taken 


100  THE   CONSTITUTION   OF    THE    NATURAL   SILICATES. 

as  double  those  of  the  pyroxenes,  for  the  reason  that  the  atomic 
replacements  seem  to  occur  by  fourths  rather  than  by  halves.  This 
point  is  exemplified  by  a  comparison  between  diopside  and  tremoHte, 
which,  reduced  to  their  simplest  empirical  formulae,  become 

Diopside CaMgSigOe 

Tremolite CaMg3Si40i2 

The  pyroxenes,  however,  are  somewhat  heavier  than  the  amphiboles 
and  from  their  greater  density  we  may  suppose  them  to  have  the 
larger  molecules.  Hence  the  formula  of  diopside  should  be  a  multiple 
of  that  just  cited  and  presumably  greater  than  Ca2Mg2Si40i2.  Upon 
this  point  the  phenomenon  of  urahtization  has  definite  bearing.  In 
this  process  pyroxene  is  converted  into  amphibole,  with  increase  of 
volume  and  little  or  no  change  of  composition.  In  other  words,  a 
complex  molecule  has  been  dissociated  into  simpler  molecules — a 
phenomenon  the  direct  opposite  of  polymerization.  In  the  face  of 
this  evidence  it  is  difficult  to  see  how  the  current  views  as  to  the 
relative  molecular  magnitudes  of  pyroxene  and  amphibole  can  be 
maintained.  The  pyroxenes  must  form  the  more  complex  group 
and  the  amphiboles  the  simpler. 

In  the  amphibole  group  the  orthorhombic  anthophylHte  is  the 
equivalent  or  isomer  of  enstatite  and  hypersthene.  Then  follows  a 
monoclinic  series,  containing  tremolite,  actinolite,  cummingtonite, 
daimemorite,  and  other  minerals,  all  represented  by  the  general 
empirical  formula  RSiOg,  with  calcium,  magnesium,  iron,  or  man- 
ganese as  the  bivalent  metal.  In  griinerite  the  salt  FeSiOg  exists  by 
itself,  and  in  richterite  and  astochite  alkaline  siUcates  appear.  If  we 
regard  the  minerals  as  pseudometasiHcates,  having  molecular  weights 
lower  than  the  pyroxenes  and  with  the  bases  replaceable  by  fourths, 
the  typical  amphiboles  are  most  simply  represented  by  formulae  like 
the  following: 

AnthophylHte.  Tremolite. 

Mg  Mg 

II  II 

Mg<  >Mg  Mg<  >Mg 

Mg  Ca 

Richterite  becomes  a  mixture  of  salts, 

R  R 

/SiO,.  /SiO,\ 

R<  >R  +  R<  >R 

\Si,0/  \Si3O, 


SILICATES  OF   DYAD  BASES.  101 

commingled  in  ratios  near  1  :  1 ,  and  astochite  is  similar,  but  with  NaH 
in  place  of  Na2.  Potassium  may  also  partly  replace  sodium.  Another 
alkaline  amphibole  of  doubtful  character,  waldheimite,  approximates 

R 

.A 


i 


which  is  the  formula  of  a  trisiHcate  pure  and  simple,  with  R  =  Ca,  Fe 
Mg.     The  existence  of  this  compound  is  strong  evidence  in  favor  of 
the  pseudometasihcate  theory,  and,  as  will  be  seen  later,  it  does  not 
stand  alone. 

Among  the  amphiboles,  as  among  the  pyroxenes,  aluminous  and 
ferric  compounds  are  common,  and  with  these  the  minerals  approach 
orthosihcate  ratios.  Tschermak's  interpretation  of  these  ratios  is 
practically  the  same  as  in  the  pyroxene  series,  namely,  by  the  assump- 
tion of  molecules  of  the  form  AlgRSiOg  or  Al^RgSijOij.  An  alterna- 
tive to  this  view  is  offered  by  Scharizer,^  who  shows  that  the  horn- 
blendes can  be  explained  as  roixtures  of  actinolite,  R4Si40i2,  with  an 
orthosihcate  called  syntagmatite  (R'2R'03Al2(SiO4)3,  whose  ratios  are 
similar  to  those  of  garnet.  An  amphibole  from  Jan  Mayen  Island 
approaches  very  nearly  to  syntagmatite  in  composition,  and  so  also 
does  the  Canadian  hastingsite.  If  a  compound  of  this  type  is  present 
in  the  amphiboles  it  would  explain  at  once  their  alterabihty  into 
epidote,  micas,  and  chlorites,  but  so  far  as  the  composition  of  the 
group  is  concerned  neither  Tschermak's  view  nor  Scharizer's  is  abso- 
lutely necessary.  The  Tschermakian  molecule,  however,  can  be 
written  either  as 

A102Mg  (A10)2 

/SiO,. 

\sio/  ^io/ 

A102Mg  (A10)2 

the  latter  form  resembling  that  of  tremoHte,  and  also  connecting  the 
group  still  more  closely  with  the  oh  vines.  It  is  also  parallel  to  the 
last  formula  suggested  for  the  corresponding  pyroxene  compound, 
being  one-half  of  that  formula  and  identical  with  it  m  type. 

No  amphibole  is  yet  known  which  corresponds  precisely  in  consti- 
tution to  acmite  and  spodumene.     In  glaucophane  we  find  a  species, 


M{         >A1  or  as 


1  Neues  Jahrb.,  1884,  vol.  2,  p.  143. 


102  THE   CONSTITUTION   OF    THE    NATURAL   SILICATES. 

which^  as  a  metasilicate,  may  be  written  AlNaSijOe  +  (MgFe)  SiOg, 
and  in  crocidolite  we  find  anotlier  similar  salt;  Fe'^'NaSisOe  +  FeSiOg. 
Crocidolite  alters  easily,  and  one  of  the  products  of  alteration,  which 
has  been  named  griqualandite,- is  very  near  Fe^'HSigOg,  the  equiva- 
lent of  acmite  in  general  type.     This  last  compound  can  be  written 

H 

^SiO,. 

■  i 

and  so  adjusted  as  an  amphibole-hydrogen-acmite  to  the  remainder  of 
the  group,  but  glaucophane  and  crocidolite  are  best  formulated  as 
follows : 

Glaucophane.  Crocidolite. 

(AIO),  (Fe"'0), 

.11      ,  .11 

<Si308\  ySigOgV 

>Mg  Fe<  >Fe 

Si,0/  \Si,0/ 


^SigOg 

Ka^ 


Na, 


which  makes  them,  as  trisilicates,  precisely  equivalent  in  structure  to 
the  normal  amphiboles.  These  compounds,  and  their  corresponding 
orthosilicates,  commingled  with  salts  Hke  tremolite  or  actinolite,  give 
mixtures  which  conform  in  composition  to  the  aluminous  hornblendes. 
Rhodusite,  which  is  alUed  to  glaucophane,  appears  to  be  a  mixed 
silicate  of  the  following  constitution: 

Na2  (Fe'''0)2 

.11  .11 

2  Fe<  >Mg  +  1  Mg<  >Mg 

^SigOs^  ^Sifi/ 

Mg  -  Mg 

In  rhodusite  the  ratio  of  Si  to  O  is  distinctly  less  than  1  to  3,  and 
the  same  is  true  of  crossite,  which  may  be  written  as  a  mixture  of  the 
two  molecules 

R  (Fe-'O), 


>R  +        iR<:      >] 

i,0/  VgO/ 


Naj  Nag 

with  some  Al  in  place  of  Fe,  and  R  being  =  Fe,  Mg,  Ca. 


SILICATES  OF  DYAD  BASES.  10^ 

Arfvedsonite,  in  which  W  is  mainly  Fe'',  is  represented  quite 
closely  as  a  mixture  of  this  order: 

Fe  Ca  Fe 

2  Fe<    '   '\Fe      +      4  Fe<    '   "\Fe     +     6  Fe/^''^'\Fe 

\sio/  \sio/  ^ao/ 


(AlO)^  Na^  .  Na, 

Hastingsite,  soretite,  and  philipstadite  may  all  be  formulated  in  a 
similar  way,  except  that  they  tend  toward  the  orthosilicate  end  of 
the  series.  In  philipstadite  SigOg  and  Si04  appear  in  equal  propor- 
tions; in  the  two  other  amphiboles  SiO^  is  largely  in  excess.  Gas- 
taldite  and  riebeckite  are  similar  species,  but  the  analyses  are  not 
perfectly  conclusive.  It  is,  perhaps,  necessary  to  assume  the  presence 
in  these  minerals  of  acmite-Uke  molecules,  riebeckite  being  empirically 
near  2  Fe'^NaSigOg  +  FeSiOg.  The  formula  of  barkevikite  is  also 
uncertain. 

Several  minerals  belonging  to  the  group  of  amphiboles  are  char- 
acterized by  the  presence  in  them  of  noteworthy  quantities  of  titan- 
ium. If  this  is  assumed  to  represent  an  orthotitanate,  the  formulae 
all  reduce  to  expressions  exactly  similar  to  those  given  in  the  preceding 
pages.  For  example,  the  type  mineral  of  this  group  is  senigmatite, 
to  which  the  following  formula  may  be  assigned: 

Fe  Fe  Fe 

/TiO.v  /SiO,\  ySisOg. 

I'        I         i 

in  which  W  is  f  Na  and  f  (ALFe''0O. 

Cossyrite  is  a  similar  mixture  of  molecules,  and  very  near 

10  Fe3(Ti04)2Na2 
7  Fe3(Si30s)3(Fe'''0)3 
10  Fe3(Si308)2Na2 

with  the  usual  variations  due  to  the  replacement  of  Fe  by  Ca,  Mg, 
etc.,  of  Na  by  K,  etc. 


104  THE   CONSTITUTION   OF   THE    NATURAL   SILICATES. 

The    other   titaniferous    amphiboles    are    also    evidently   variable 
mixtures,  which,  in  the  best  analyses,  are  representable  as  follows: 

Linosite.  Kxrsutite. 

1  Ca^FeCTiOJ^Na^  2  CaFe^CTiOJ^Na^ 

1  Ca^MgCSiOJ^CFe'^'O^  2  Ca^MgCSiOJ^HCAlO) 

2  Mg3(Si303)2(A10),  3  Mg3(Si303)2(A10)3 

Anaphorite.  Rhonite. 

1  R3(TiO,)2Na2  2  Mg,Ca(TiOj2(RO)2 

3  R3(Si30s)2Na2  5  Mg^CaCSiOJ^CRO)^ 
1  R3(Si303)2(Fe'''0)2  5  Fe2Ca(SiOj2(RO)3 

In  anaphorite  R=Ca,  Fe,  Mg,  in  the  ratio  1  :  2  :  5;  and  in  rhonite 
RO  is  about  §  AlO  to  J  Fe'^'O.  Rhonite  represents  the  orthosilicate 
end  of  the  series ;  anaphorite  shows  the  nearest  approach  to  trisilicate 
ratios.  It  is  evident  that  an  indefinite  number  of  similar  molecular 
mixtures  are  possible,  but  all  are  likely  to  be  of  the  same  general  type. 
A  careful  study  of  the  best  analyses  in  the  pyroxene  and  amphibole 
groups  will  strengthen  very  materially  the  view  here  developed  that 
the  species  are  not  true  metasilicates.  Although  in  most  of  the 
analyses  the  approximation  to  metasilicate  ratios  is  very  close,  there 
are  distinct  variations  toward  orthosilicates  on  one  side  and  toward 
trisilicates  on  the  other,  and  it  is  only  by  assuming  that  we  have 
mixed  silicates  to  deal  with  that  all  the  anomalies  can  be  made  to 
disappear.^  On  this  theory,  if  we  represent  SigOg,  SiO^,  and  Ti04 
groups  indiscriminately  by  the  general  symbol  X,  all  the  amphiboles 
are  covered  by  the  following  typical  symbols,  in  which  R"  stands  for 
any  dyad  metal,  and  R'  for  K,  Na,  H,  AlO,  or  Fe'"0: 

R''A 


R"3X,R', 

or,  structurally, 

R"AR', 

R" 

A 

II 

R" 

A 

E'. 

1 

II 

r'^' 

R', 

A 

In  a  similar  way  all  the  pyroxenes,  except  the  acmite-spodumene 
group,  which  has  the  special  formula  discussed  previously,  may  be 
represented  as  formed  by  mixtures  of 

R^^X^R'', 
R",X,(R-'0)s 

1  This  view'of  the  constitution  of  the  pyroxenes  and  amphiboles  was  first  advanced  by  G.  F.  Becker, 
Am.  Jour.  Sci.,  3d  ser.,  vol.  38,  p.  154, 1889. 


SILICATES  OF   DYAD  BASES.  105 

which  is  i^  accordance  with  the  theory  developed  by  Tschermak  except 
as  to  the  molecular  magnitude  of  the  compounds— that  is,  the 
pyroxenes  are  essentially  bipolymers  of  the  amphiboles,  and  the 
character  of  the  structure  is  the  same  for  both  groups.  The  analogy 
between  these  formulae  and  those  of  the  oUvines  has  already  been 
pointed  out,  and  it  is  emphasized  by  still  more  evidence.  Pseudo- 
morphs  of  pjrroxene  (fassaite)  after  monticellite  have  been  found  at 
Monzoni  and  are  well  known.  Furthermore,  Becke  ^  has  described 
pseudomorphs  of  anthophyllite  and  actinolite  after  olivine,  so  that  a 
connection  between  the  two  groups  is  clearly  indicated.  The  tracing 
of  this  connection  in  a  more  general  way  would  seem  to  offer  a  profit- 
able field  for  investigation. 

Many  amphiboles  contain  water  and  some  contain  fluorine.  These 
constituents  are  easily  accounted  for,  being  present  either  as  univalent 
radicles  like  AlOgHj  and  AIF2,  or  with  H  replacing  Na.  Allen  and 
Clement,^  however,  in  their  study  of  tremolite  found  that  the  mineral 
contained  water  which  could  be  expelled  continuously  and  therefore 
behaved  as  if  it  were  not  constitutional  but  in  "solid  solution."  A 
study  of  their  analyses  leads  to  some  doubt  as  to  this  conclusion.  The 
water  as  determined  is  needed  to  completely  satisfy  the  silica,  and 
so  to  give  rational  formulae  to  the  different  samples  of  tremoUte 
which  they  studied.     Additional  investigation  is  plainly  needed  here. 

Another  view  of  the  amphiboles  has  been  developed  by  Penfield 
and  Stanley,^  who  assume  the  presence  in  them  of  such  bivalent 
groups  as  Al^OF^,  AipCOH)^,  AlA^^'',  and  Al^O^R^Na^,  and  also 
the  univalent  group  MgF.  With  the  aid  of  such  assumptions  they 
are  able  to  formulate  all  the  amphiboles  as  metasificates.  Such  an 
interpretation  of  these  minerals  is  evidently  more  complicated  than 
the  formulation  adopted  here,  and  it  does  not  provide  for  the  varieties 
in  which  the  ratio  Si  to  O  is  below  the  metasilicate  requirements. 

By  the  hydration  of  pyroxene  or  amphibole  either  serpentine  or  talc 
may  be  generated.  Talc  has  the  composition  HaMggSiPia,  and  may 
be  written  structurally  like  amphibole  either 

Mg  ^  .  H, 

A  A 

Both  expressions  are  in  accord  with  the  fact  recorded  by  Schneider 
and  myself,^  that  upon  the  ignition  of  talc  one-fourth  of  the  silica  is 

1  Min.  pet.  Mitt.,  vol.  4,  pp.  355,  450,  188l-«2. 

2  Am.  Jour.  Sci.,  4tli  ser.,  vol.  26,  p.  101, 1908. 
a  Am.  Jour.  Sci.,  4th  ser.,  vol.  23,  p.  23,  1907. 

4  U.  S.  Geol.  Survey  Bull.  78,  p.  13, 1891. 


106  THE  CONSTITUTION  OP  THE  NATUEAL  SILICATES. 

set  free  quantitatively.  This  would  give  the  ignited  residue  the  com- 
position shown  by  the  subjoined  alternative  formute: 

Mg 

II 
/SiO,.  ^O 

Mg^  >Mg  and  Mg\  >Mg  . 

\Si,0/  ^hO/ 

II 
Mg 

and  of  these  the  first  would  seem  to  accord  the  better  with  the  remark- 
able stability  and  insolubility  of  the  material.  A  metasilicate  for- 
mula, H2Mg3(Si03)4,  is  also  admissible,  and  accords  equally  well  with 
the  evidence  concerning  talc.  The  pseudometasilicate  expression, 
however,  seems  to  be  preferable  in  view  of  what  is  known  as  to  the 
genesis  of  the  species. 

The  nickel  silicate,  alipite,  HgNig (8103)3,  also  has  metasilicate  ratios, 
but  there  is  no  further  evidence  as  to  its  constitution. 

ChrysocoUa  is  probably  a  metasilicate,  and  perhaps  empirically 
CuSi03.2H20.  It  can  not  be  well  regarded  as  impure  dioptase,  for  that 
mineral  gelatinizes  with  hydrochloric  acid,  whereas  chrysocolla  does 
not.  The  species,  which  may  be  a  mixture  of  compounds,  needs 
careful  investigation.  The  same  is  true  of  plancheite,  to  which 
Lacroix^  assigns  the  complex  formula  HgCuyCCuOH)  8(8103)13.  Too 
little  is  known  of  this  species  to  admit  of  any  more  definite  formu- 
lation. 

The  lead  silicate  alamosite,  Pb8i03,  is  analogous  to  wollastonite 
and  is  therefore,  in  all  probability,  a  true  metasilicate.  Agnolite, 
HaMug  (8103)4.1120,  resembles  talc  in  its  ratios,  and  may  also  be 
classed  here. 

Leucophanite,  NaCaGlF8i206,  is  another  definite  species  which  is 
easily  figured  thus : 

/SiOg— Gl— F  /8i03— Ca— F 

Ca<  or  Gl< 

\8iO3— Na  \8iO3— Na 

but  between  the  two  alternatives  there  is  no  way  of  deciding. 

The  mineral  hillebrandite  was  interpreted  by  its  discoverer, 
Wright,^  as  an  orthosilicate  of  calcium,  Ca28i04.H20.  The  water, 
however,  is  not  given  off  at  low  temperatures,  and  is  probably  con- 
stitutional.    If  so,  the  species  is  a  basic  metasilicate, 


/Ca— OH 
OH 


8i03< 


1  Compt.  Rend.,  vol.  146,  p.  722,  1908;  Soc.  min.  Bull.,  vol.  31,  p.  250,  1908. 

2  Am,  Jour.  Sci.,  4th  ser.,  vol.  26,  p.  551, 1908. 


SILICATES  OF   DYAD  BASES.  107 

This  conclusion  is  borne  out  by  the  fact  that  the  mineral  gives  an 
immediate  and  deep  rose  color  when  its  powder  is  moistened  with 
phenolphthalein  solution.  That  reaction  is  suggestive  of  the  alkahne 
group  CaOH,  and  a  new  fluosihcate,  custerite,  recently  discovered  by 
Umpleby  and  SchaUer,  of  the  Geological  Survey,  gives  the  same 
coloration.     Custerite  is  represented  by  the  formula 

/Ca— F 
Si03< 

x:;a— OH 

which  is  analogous  to  that  proposed  for  hillebrandite. 

Several  other  species  are  possibly  metasilicates,  although  the  evi- 
dence is  not  sufficient  to  warrant  a  definite  conclusion.  They  are 
empirically — 

Weinbergerite NaFe^^gAlSi^Oig 

Spodiophyllite (Na,K)2(Mg,Fe)3(Al,Fe)2Si8024 

TaramelUte Ba4Fe^^Fe^^^4Siio03i 

PhoHdohte,  a  hydrous  magnesian  sihcate,  containing  some  potassium 
and  aluminum  also,  seems  to  admit  of  no  simple  and  satisfactory 
formula. 

DISILICATES  AND  TRISILICATES. 

Although  the  existence  of  the  sexbasic  acid  H6Si207  has  been  well 
estabhshed  by  the  preparation  of  its  esters,  its  metallic  salts  are  Httle 
known  and  uncertain.  I  have  already  shown  (see  discussion  of 
serpentine,  p.  92)  that  a  mineral  may  be  apparently  an  orthodisilicate 
and  yet  be  equally  well  explainable  otherwise;  and  what  is  true  for 
that  mineral  may  be  true  for  others.  For  the  following  species  the 
orthodisihcate  formulae  seem  to  be  the  best  and  simplest,  even,  though 
they  are  not  wholly  free  from  objection.  They  fit  existing  evidence 
but  are  not  absolutely  conclusive. 

The  typical  member  of  this  group  of  minerals  is  the  hexagonal  lead 
silicate,  barysihte,  PbgSiaO^.  The  artificial  compound  from  the  slags 
of  Bonneterre,  Missouri,  described  by  Dana  and  Penfield,  is  near  this 
in  composition,  and  may  be  PbaCaSiaOy.  Hardystonite,  Ca2ZnSi207, 
is  also  typical.  The  pseudodeweylite  of  Zambonini,^  which  has  the 
formula  Mg3Si207.3H20,  is  another  compound  of  this  type.  The 
formulae  assigned  to  these  species  are  merely  their  simplest  empirical 
expressions.  The  true  molecular  weights  are  not  known  nor  is  there 
any  available  evidence  to  show  whether  or  no  the  formulae  should  be 
doul)led  or  tripled. 

Two  other  lead-calcium  silicates  appear  to  belong  here.  Nasonite, 
described  by  Penfield  and  Warren,^  contains  chlorine,  and  ganomalite 

1  Contributo  alio  studio  dei  silicati  idrati,  p.  88, 1908. 

2  Am.  Jour.  Sci.,  4tli  ser.,  vol.  8,  p.  346, 1899. 


108  THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 

is  regarded  by  them  as  the  corresponding  hydroxyl  compound.  The 
formulae  are  simply  written  as  follows : 

Nasonite.  Ganomalite. 

/Si^O^.Ca^CPbCl)  ySiA-Ca^CPbOH) 

Pb<  Pb< 

>iA.Pb2  >i207.Pb2 

Pb<  Pb< 

\SiA-Ca2(Pba)  \SiA-Ca2(PbOH) 

The  univalent  groups  — Pb — CI  and  — Pb — OH  are  similar  to  other 
groups  which  have  been  assumed  elsewhere. 

The  group  of  zeolitic  calcium  silicates,  okenite,  gyrolite,  and  apo- 
phylUte,  are  unquestionably  related  to  one  another  and  are  best 
represented  as  salts  of  IleSiaOy.  In  nature  gyrolite  may  be  derived 
from  apophyllite  and  apophyllite  also  from  gyrolite,  and  Doelter  has 
generated  apophyllite  from  okenite  by  artificial  means.^  First,  by 
heating  okenite  with  potassium  silicate  and  water  to  200°,  crystals  of 
apophylhte  were  obtained.  Secondly,  by  heating  okenite  with 
aluminum  chloride,  sodium  carbonate,  and  carbonated  water  at  220°, 
apophyllite,  analcite,  and  chabazite  were  produced.  The  most  satis- 
factory general  formulae  for  the  three  species  are  these: 

Okenite.  Gyrolite.  Apophyllite. 

/SiA-Hs  /Si^O^.CaHg  /Si207.H4(CaOH) 

Ca<  Ca<  Ca< 

^Si^O^.CaH^  >Si207.H4  ^Si^O^.H^ 

Ca<  .  Ca<  Ca< 

^Si^O^.Hs  \Si2O7.CaH3  \Si207.H,(CaOH) 

In  apophyllite  fluorine  may  partly  replace  hydroxyl,  and  K  may 
replace  the  univalent  CaOH.  With  K  :  CaOH  :  :  1  :  1  the  composition 
of  apophyllite  becomes 

SiOg: - 52.03 

CaO 24.  27 

K2O 6.  79 

H2O 16.  91 

100.  00 

The  uncertain  mineral  plombierite  may  be  a  fourth  member  of  this 
group,  with  the  formula 

.Si^O.-CaHa 
Ca< 

>SiA-Ca2      +9H2O 
Ca'^ 

\si2O7.CaH3 

1  Neues  Jahrb.,  1890,  vol.  1,  p.  118. 


SILICATES  OF   DYAD  BASES. 


109 


To  the  calcium-manganese  silicate,  inesite,  various  formulse  are 
assignable.  By  Flink  it  is  regarded  as  2(CaMn)Si03.H20.  But  part 
of  the  water  is  stable  at  temperatures  above  300°,  and  this  fact  is 
expressed  by  Schneider's  formula  (CaMn)Si308(MnOH)2.H20,  Both 
formulae  agree  with  the  analyses  approximately,  but  the  analysis  by 
Lundell  is  better  represented  by  the  following  mixture: 


/SigOy.CaHg 
Ca<  Mn< 

^Si^O^.Ca^      +2H2O,  and 
Ca<  Mn< 

\Si2O7.CaH3 


ioO^.MnH, 


5i207.Mn2      +  2IL0 


^iaOy.MnHg 

which  requires  the  following  percentage  composition: 


Found, 
Lundell. 

Calculated. 

SiOo 

42.92 

.73 

36.31 

.37 

8.68 

10.48 

42  18 

PbO 

1            37.44 

1             9.84 
10.54 

MnO  ..                                 

MffO...            

CaO 

H2O 

99.49 

■ 

100.00 

To  inesite  from  Mexico,  which  contains  less  water  than  is  shown 
above,  Farrington  ^  assigns  the  formula  HaC^In,  Ca)6Si60i9.3H20. 
This  formula  is  difficult  to  represent  constitutionally  and  does  not  fit 
the  analysis  as  sharply  as  is  to  be  desired.^ 

A  similar  structure  probably  belongs  to  the  magnesian  spadaite, 

as  follows: 

.Si20,.MgH3 
Mg< 

>Si207.MgH2 
Mg< 

\Si20,MgH3 

Another  magnesian  silicate,  saponite,  is  perhaps  normally  H4(MgOH)3 
Si207,  although  the  analyses  all  show  admixtures  of  some  aluminous 
compound. 

To  cuspidinc,  a  calcium  fluosilicate,  Zambonini  ^  assigns  the  sub- 

I'oined  formula:  ,  _ 

^  /Ca— F 


1  Field  Columbian  Mus.  Bull.,  Geol.  ser.,  vol.  1,  p.  221, 1900. 

2  See  also  Zambonini,  loc.  cit. 


»  Mineralogia  vesuviana,  p.  273, 1910. 


110  THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 

An  analysis  of  cuspidine  from  Franklin,  New  Jersey,  by  Warren  leads 
to  the  same  formula,  although  Palache  ^  interprets  it  differently. 
Zambonini,  following  Groth,  regards  bertrandite  as  similar,  and 
assigns  to  it  the  formula 

Gl^Sip^.CGlOH)^. 

In  accordance  with  Hillebrand's  analysis,^  the  rare  mineral  row- 
landite  should  have  an  orthodisilicate  formula  as  follows: 

Y— F 

/Sip,=Y 
Fe< 

\Si207=Y 

Y— F 

which  expresses  the  composition  of  the  species  very  closely. 

Minerals  corresponding  to  metadisilicic  acid,  HaSisOg,  are  rare.  A 
few  of  them  have  already  been  mentioned  among  the  alumosilicates, 
but  only  two  belong  here.  Rivaite,  described  by  Zambonini,^  is  a 
metadisihcate  of  calcium  and  sodium  and  is  probably  to  be  figured 

^iaOg— Na 


which  suggests  a  relationship  withpectohte. 

The  rare  mineral  hyalotekite,  which  contains  boron,  agrees  very 
closely  with  the  empirical  formula  R^BFSieOjy,  if  we  regard  the  water 
in  it  as  hydroxyl  replacing  fluorine.  This  can  be  interpreted  either 
as  a  metadisihcate  or  as  a  trisiHcatC;  with  the  following  alternative 
formulae : 

NSiPs  or  Il<  yR 

\Si2O  — R— F  0=B/  \R— F 

Its  association  with  feldspar  and  schefferite  rather  favors  the  trisili- 
cate  formula,  but  the  two  are  empirically  identical.     If  we  reduce 

1  Am.  Jour.  Sci.,  4th  ser.,  vol.  24,  p.  185, 1907         3  Appendice  alia  mineralogia  vesuviana,  p.  16. 

2  U.  S.  Geol.  Survey  Bull.  113,  p.  45, 1893. 


SILICATES  OF   DYAD  BASES. 


Ill 


Lindstrom's  analysis  to  100  per  cent,  after  calculating  the  water 
(ignition)  into  its  equivalent  of  fluorine,  rejecting  as  impurities  the 
traces  of  Mfi,  and  Fefi,,  and  consolidating  hke  bases,  we  get  the 
following  comparison  with  theory: 


Found. 

Reduced. 

Calculated. 

SiOa 

39.47 
3.73 

25.11 
.09 
.29 
.75 
.17 
7.82 

20.08 
.09 
.89 
.18 
.06 
.99 
.06 
.59 

38.60 

3.63 

1          25. 71 

J-          9.43 
1          21. 32 

. 

B2O3 

38. 10 

pbo : ■••■ 

3.71 
26.22 

CuO 

MnO 

GIO • 

Na20 

CaO 

9.22 
21.5^ 

BaO 

MgO 

k;o :: 

A1203 

FeA 

F.!... 

2.26 

CI 

2.01 

Ignition 

LessO 

100.37 

100.  95 
.95 

100. 85 
.85 

100.00 

100.00 

In  computing,  R"  has  been  regarded  as  Ca  :  Ba  :  Pb  :  :  7  :  6  :  5; 
that  is,  hyalotekite  is  a  mixture  of  isomorphous  calcium,  barium,  and 
lead  salts  in  the  indicated  ratio.  The  agreement  between  analysis 
and  theory  is  as  close  as  could  be  reasonably  expected. 

Salts  of  the  octobasic  orthotrisilicic  acid  HgSigOio  seem  to  be  few  in 
number,  at  least  so  far  as  present  evidence  goes.  Two  of  them  are 
silicates  of  nickel,  namely, 

Connarite HaNijSiaOio 

Genthite MgaNiaSiaOio.eHjO 

Deweylite,  according  to  Zambonini,^  is  equivalent  to  genthite,  its 
formula  being  MgSi30io.5-6H20.  He  shows  that  the  so-called  dewey- 
lite reaUy  represents  two  species,  one  having  the  formula  just  given, 
the  other,  pseudodeweylite,  being  an  orthodisilicate.  Pseudodewey- 
Hte  has  already  been  considered. 

The  complex  fluosihcate,  meliphanite,  is  also  probably  an  orthotri- 
silicate,  although  other  formulae  have  been  proposed  for  it.     Zeophyl- 

1  Contributo  alio  studio  dei  silicati  idrati,  p.  88, 1908. 


112 


THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 


lite,  which  is  in  some  respects  aUied  to  apophylUte,  belongs  in  this 
class,  and  the  two  minerals  appear  to  be  similar  in  structure,  thus: 


Meliphanite. 

/O— Na 
Si— O. 


0 


Si 

i 


\Ca 


Zeophijllite. 

Si— Ov 

0 

I  /O— Ca— F 
Si<  +H,0 

I  \0— Ca— F 


Gl 


sko>- 


\, 


Si— o. 

O— Ca— F  O— H 

Trisilicates  of  the  form  H^SigOg  are  numerous  in  the  mineral 
kingdom  and  are  especially  represented  by  the  alkali  feldspars  and 
their  derivatives.  They  are  common  in  isomorphous  admixture  with 
orthosilicates,  forming  the  minerals  which  I  have  classed  as  pseudo- 
metasiUcates,  and  only  two  remain  to  be  noted  here.  These  two 
silicates,  eudidymite  and  epididymite,  have  the  same  empirical  for- 
mtda,  HNaGlSigOg,  which,  doubled,  becomes 


Na2 
/Si.O, 


Gl 


^.. 


%g: 


Si,0/ 


1 


a  structure  conformable  to  the  type  of  several  orthosilicates.  The 
isomerism  between  eudidymite  and  epididymite  is  explainable  by 
giving  one  species  the  constitution  just  written,  whereas  the  other,  in 
place  of  Na2  and  H2,  would  have  the  two  groups  NaH.  It  can  also 
be  ascribed  to  a  different  linking  with  the  oxygen  of  the  acid,  and  the 
empirical  formula  HNaGlSigOg  can  be  put  in  two  forms,  thus: 
0=Si— O— Na  Si— O 


0=Si— O— H 


and 


o 


r°- 


-Na 


0=Si— O— H 


a  third  isomer  being  also  conceivable.  The  doubled  formula,  however, 
brings  out  analogies  with  bertrandite  and  other  species  and  therefore, 
in  default  of  evidence,  is  to  be  preferred. 


CHAPTER  V. 

SIIilCATES  OF  TETRAD  BASES,  TITANO SILICATES,  AND 
COLUMBOSILICATES. 

On  account  of  their  relatively  small  number  and  general  scarcity, 
the  sihcates  containing  the  tetrad  metals,  tinanium,  zirconium,  and 
thorium,  are  difficult  to  interpret  in  any  satisfactory  manner.  Evi- 
dence exists,  however,  which  seems  to  show  that  they  are  explainable 
by  the  same  principles  which  apply  to  aluminum  and  the  dyads,  and 
that  the  theory  of  substitution  from  normal  salts  is  a  good  working 
hypothesis  to  start  from. 

One  definite  normal  salt  is  known  in  this  series,  the  mineral  zircon, 
ZrSiO^.  As  with  the  other  inorganic  sihcates,  the  true  molecular 
weight  of  this  compound  is  unknown,  and  it  can  be  inferred  only  from 
a  study  of  its  derivatives.  If  we  assume  it  to  be  represented  by  the 
polymeric  expression  Zr4(Si04)4,  it  contains  replaceable  basic  atoms, 
and  a  number  of  other  zirconium  sihcates  fall  naturally  into  series 
derivable  from  this  as  the  fundamental  member.  In  this  connection 
the  mineral  auerbachite  is  pecuharly  suggestive,  for  its  composition 
is  best  indicated  by  the  formula  Zr4(Si308)(Si04)3,  which  goes  to  show 
an  important  analogy  between  this  group  of  silicates  and  those  which 
have  been  previously  considered.  This  formula,  compared  with 
Hermann's  analysis  of  auerbachite,  gives  the  foUowing  results: 


Hermann. 

Calculated. 

SiOo 

42.91 

55.18 

.93 

.95 

42.45 

ZrOa                                                                 

57.55 

FeO                                                           

HgO                                                   

99.97 

100.00 

Although  zircon  is  a  very  stable  and  defuiite  mineral,  it  alters  by 
hydration  into  malacone,  cyrtohte,  and  a  variety  of  other  indefinite 
substances  which  can  not  as  yet  be  interpreted  with  any  clearness. 
At  the  same  time  other  bases,  such  as  lime  and  the  rare  earths,  are 
taken  up,  producing  mixtures  of  great  complexity.  Malacone 
43633°— Bull.  588—14 8  113 


is 


114  THE   CONSTITUTION    OF   THE   NATURAL   SILICATES. 

probably  the  first  hydration  derivative,  and  its  relations  to  zircon, 
regarding  the  latter  as  Zr4(Si04)4,  may  possibly  be  as  follows: 

Zircon.  Malacone. 

Si04=Zr  OH 

/siO  /  /siO  ~^ 

Zr<^.^^^Zr  Zr<     ^^Zr 

V        S1O4.  \      SlO^K 

Si04=Zr  Si04=Zr 

The  original  cyrtohte  from  Rockport  is  near  malacone,  but  the 
cyrtolite  from  Colorado,  analyzed  by  Hillebrand,  approximates  to 


Zr 


OH 

OH 

SiO^^HCZrO^H^) 

Si04=H(Zr02H2) 


with  part  of  the  ZrOgHg  replaced  by  other  bases.  A  number  of  other 
altered  zircons  or  derivatives  of  zircon  have  received  specific  names, 
but  their  nature  is  more  or  less  doubtful.  Two  of  them,  however,  are 
interesting  and  may  represent  distinct  species.  The  anderbergite  of 
Blomstrand,  for  instance,  is  very  near 

'SiO.=Zr— OH 


^    /Si04=Y 


.Si04=Zr— OH 

and  the  alvite  of  Nordenskiold,  analyzed  by  Lindstrom,  is  well  repre- 
sented as  a  molecular  mixture  of  two  compounds, 

Zr(SiOj4(ZrOH)4  +  Zr(SiOj4(G10H)i2. 

The  alvite  of  Forbes  and  Dahll  seems  to  be  quite  different,  but  the 
analysis  of  it  is  unsatisfactory.  In  Lindstrom' s  analysis  one  ZrOH 
group  is  replaced  by  R'",  mainly  Fe,  Y,  and  Ce,  and  a  little  GIOH  is 
replaced  by  Ca.  Many  altered  zircons,  like  these  minerals,  contain 
rare  earths  in  greater  or  less  proportion,  and  this  fact  counts  heavily 
against  a  common  interpretation  of  zircon  as  a  mixed  oxide  having 
the  formula  ZrOg  4  SiOj,  or 

0=:Zr/   \si=0 

This  view,  which  is  prevalent  in  Germany,  is  based  upon  the  close 
morphologic  relations  between  zircon,  rutile,  and  cassiterite.     The 


SILICATES  OF   TETRAD  BASES.  115 

chemical  relations,  however,  are  at  least  equaUy  important,  and  a 
good  formula  must  express  or  suggest  them  all.  The  formula  here 
proposed  for  malacone  is  not  absolutely  certam.  The  dehydration 
experiments  of  Zambonmi '  seem  to  show  that  it  may  be  merely  a 
hydrated  zircon,  that  is,  a  zircon  contamuig  dissolved  water  in  variable 
amounts.  A  broader  study  of  malacone  from  several  distinct  locaH- 
ties  seems  to  be  desirable. 

The  foregoing  formulae,  taken  by  themselves,  are  entitled  to  little 
consideration,  but  they  become  more  significant  when  studied  in 
connection  with  other  compounds  later.  It  wHl  be  noticed  that  one 
atom  of  zirconium  is  represented  as  linking  four  groups  or  radicles 
together ,^  just  as  one  aluminum  atom  has  a  similar  triple  function  in 
the  aluminous  orthosilicates.  This  is  practically  equivalent  to  regard- 
ing the  minerals  under  consideration  as  derivatives  of  a  complex  dode- 
cabasic  zircosihcic  acid,  H^^ZrSifi^^,  which  is  at  least  as  probable  an 
interpretation  as  any  other  which  has  so  far  been  advanced. 

In  eudialyte,  elpidite,  and  catapleiite  we  have  a  group  of  zirconium 
sihcates  which  form  a  highly  suggestive  series.  Taking  the  simpler 
members  first,  they  may  be  represented  thus: 


Elpidite. 

Lime  catapleiite. 

Soda  catapleiite. 

/OH 
/VOH 

/OH 
//OH 

/OH 
//OH 

Zr 

Zr 

Zr 

XXSigOg^Nall^ 
\si303=NaH, 

\\  OH 
\Si308=CaN"a 

\\0H 
\Si303=Na3 

Connecting  these  formulae  with  zircon,  we  have  the  facts  that  Si308 
occurs  in  auerbachite,  and  that  at  Laven,  according  to  Brogger,^ 
zircon  is  found  both  intergrown  with  catapleiite  and  pseudomorphous 
after  it. 

Here  again  a  different  interpretation  of  the  minerals  has  been 
advanced  by  Zambonini,^  who  regards  their  hydration  as  extrinsic 
and  not  constitutional.  To  catapleiite  he  assigns  the  formula 
Na2ZrSi309.2H20,  which,  interpreted  constitutionally,  becomes 

0=Zr=Si308=Na2.2H20. 

The  calcium-tin  sihcate,  stokesite,  is  similar,  and  it  has  the  formula 

CaSnSi309.2H20.     Elpidite,   according  to  Zambonini,  is  a  metadi- 

silicate, 

Na^ZrSieOis.SHp. 

These  formulae  are  legitimate  and  rational,  and  the  only  valid  objec- 
tion to  them  is  that  they  fail  to  bring  out  clearly  the  relations  between 
the  several  species  and  the  fundamental  compound,  zircon. 


1  Contributo  alio  studio  dei  silicati  idrati,  p.  72, 1908. 
3  Zeitschr.  Kryst.  Min.,  vol.  16,  p.  105, 1890. 
»  Op.  cit.,  pp.  54-64. 


116  THE   CONSTITUTION   OF   THE   NATUKAL   SILICATES. 

Eudialyte  and  eucolite  are  commonly  regarded  as  metasilicates, 
with  the  compound  ZrOClj  as  an  admixture.  But  that  compound  is  not 
found  in  nature  by  itself,  and  both  minerals,  unlike  most  of  the  true 
metasilicates,  gelatinize  with  acids.  Furthermore,  the  analyses  of 
eudialyte  and  eucolite  show  a  considerable  range  of  variation  in  the 
ratio  Si  :  O,  although  approximating  somewhat  nearly  to  the  assumed 
SiOg.  If  now  we  treat  eudialyte  and  eucolite  as  mixtures  of  trisilicates 
and  orthosilicates,  like  the  feldspars,  scapolites,  and  some  micas,  all 
difficulties  vanish,  the  chlorine  becomes  equivalent  to  hydroxyl,  and 
the  minerals  fall  into  line  with  catapleiite  and  elpidite  as  the  firet 
members  of  the  series.  All  varieties  of  eudialyte  and  eucolite  are 
then  interpretable  as  mixtures  of  the  two  molecules 

CI  CI 

/siO,=CaNa  /sigO^-CaNa 

^SiO,=CaNa  ^^                 "".   SigO^^CaNa 

Si04=CaNa  SigOg^CaNa 

commingled  in  ratios  nearly  but  not  exactly  1:1.  Hydroxyl  replaces 
chlorine  to  some  extent,  while  iron  and  manganese  partly  replace 
calcium;  but  the  ratios  shown  by  the  formulae  are  constant,  and  the 
structural  analogies  with  the  aUied  species  are  perfectly  clear. 

The  so-called  ''zircon  pyroxenes,"  rosenbuschite,  wohlerite,  laven- 
ite,  guarinite,  and  hiortdahUte  all  conform  to  the  type  of  expressions 
adopted  here,  although  the  analytical  data  are  too  scanty  to  yield 
positive  conclusions.  They  can  be  given  metasihcate  ratios,  follow- 
ing Brogger,  by  regarding  the  zirconium  present  in  the  form  of  a  meta- 
zirconate.  If  this  explanation  is  correct  we  should  expect  to  find 
zirconates  in  nature,  free  from  admixtures,  but  no  such  minerals  are 
yet  known.  Artificial  zirconates  have,  indeed,  been  prepared;  but 
zirconium  is  more  markedly  basic  than  acid  in  its  functions,  and  the 
analogy  furnished  by  the  orthosiHcate  zircon  has  been  my  guide  in 
the  interpretation  of  these  species.  Furthermore,  the  ratio  Si  to  O 
in  each  mineral  is  1  to  4  or  nearly  so,  which  places  them  among  the 
orthosiUcates. 

The  simplest  of  these  species,  and  probably  the  most  definite,  are 
rosenbuschite  and  guarinite.  Guarinite,  which  originally  was 
described  as  an  isomer  of  titanite,  has  recently  been  reexamined  by 
Zambonini  and  Prior  ^  and  found  to  be  identical  or  nearly  so  with 
hiortdahlite,  although  the  two  minerals  differ  somewhat  in  composi- 
tion. Zambonini  and  Prior  represent  guarinite  as  a  mixed  salt  of 
calcium  metasilicate  and  sodium  zirconate,  with  some  calcium 
fluoride,  but  the  following  formula,  which  gives  the  same  ratios,  is 
more  probable: 

J  Mineralog.  Mag.,  vol.  15,  p.  259, 1909. 


SILICATES   OF   TETRAD  BASES.  117 

^i04=Ca3=Si04v 
F— Zr— Si04=Ca3=SiO,— Zr— F 
\si04=:Ca2=Si0/ 

Na  Na 

In  hiortdahlite,  as  represented  by  Cleve's  analysis,  the  same  molecule 
appears,  plus  a  less  condensed  molecule : 

/SiO,=Ca3=SiO,. 
F^^Zr/  >Zr-F3 

\SiO— Ca— Sio/ 


Na 


k 


NaH 


the  two  being  commingled  in  the  ratio  1:1.     Hydroxyl  may  replace 
fluorine  to  some  extent  in  either  case. 

These  formulae  compare  with  the  actual  analyses  as  follows: 


Guarinite. 

Hiort^a-hlite. 

Found. 

Calculated. 

Found. 

Calculated. 

SiOa 

30.53 
19.70 

31.81 
21.44 

31.60 

21.48 

1.50 

30  56 

ZrOo 

1            24. 77 

TiOs 

CboO.. 

1.68 
.21 

TaoO- 

FeO:        ......       -. 

.34 
.94 
.96 
32.53 
.10 

Fea. 

1.91 

1.56 

35.80 

.57 

.43 

6.13 

r          39. 36 

MnO 

CaO 

f            34. 11 

MffO 

K2O                                    

NaoO 

5.45 

6.53 

.58 

5.83 

6.30 

H,0 

.91 

F._: 

1.28 

3.34 

5.78 

Less  0                    

99.80 
.54 

101.  40 
1.40 

102.  39 
2.45 

102.  43 
2.43 

99.26 

100.00 

99.  94 

100.00 

To  rosenbuschite  the  following  formula  may  be  assigned: 

.Si04=Ca3=Si04v 
F_Zr— Si04=Ca3=SiO— Zr— F 

\siO— Ti— Sio/ 

)        i       J 
Nao      Fj      Na2 


118  THE   CONSTITUTION   OF   THE    NATUKAL   SILICATES. 

This  composition  compares  with  Cleve's  analysis  as  follows : 


Found. 

Calculated. 

SiOa 

31.36 

20.10 

6.85 

1.00 

.33 

1.39 

24.87 

9.93 

5.83 

30.30 

ZrOs 

20.54 

TiOa                                                       

6.73 

FegOg                  

LaoOg 

MnO 

• 

CaO 

28.29 

NaaO 

10.44 

F                            ..                              

6.40 

Less  0 

101.  66 

2.47 

102.  70 
2.70 

99. 19 

100.  00 

The  formulation  of  wohlerite  is  complicated  by  the  presence  of  the 
quinquivalent  element  columbium,  which  may  be  either  in  the  dyad 
group,  =CbOF,  or  in  the  trivalent,  ^Cb=0.  This  alternative  leads 
to  two  possible  types  of  structure,  one  of  them  Hke  those  immediately 
preceding,  the  other  resembling  that  of  eudialyte,  thus: 

SiO^^Ca  Si04=Ca 

/       \cbOF  /       \zrF2 

I.  4  Zr— SiO^^Ca      +  1  Zr— SiO^^Ca 

\\Si04=CaNa        \NsiO,=CaNa 
SiO^^CaNa  SiO^^CaNa 

with  small  amounts  of  the  groups  W'^  replacing  ZrFg. 

.Si04=Ca3=Si04. 
4  0=Cb— Si04=Ca3=Si04— Cb=0 
\si04=Zr  =SiO  / 

Na  Na 

n.  + 

.SiO^^Cag^SiO^v 
1  F— Zr— SiO^^Cag^SiO  — Zr— F 
\siO4— Zr— Sio/ 

Na2      F2     Na2 

On  comparing  these  alternative  formulae  with  Cleve's  analysis  of 
wohlerite  we  have: 


SILICATES  OF  TETRAD  BASES. 


119 


Found. 

Calculated. 

I. 

II. 

SiO. 

30.14 

.42 

16.12 

12.85 

.48 

.66 

1.26 

1.00 

.12 

26.97 

7.50 

.74 

2.98 

30.27 

}          18.46 

13.52 

30.48 

TiOa 

ZrOa 

19.20 

CboO, 

12.87 

FeaOa 

CegOg 

FeO 

28. 26 
7.83 

MnO 

MgO 

28.39 

CaO 

NaoO 

7  47 

H2O 

F. 

2.87 

2  75 

LessO 

101.  24 
1.24 

101.  21 
1.21 

101. 16 
1.16 

100.00 

100.00 

100.00 

These  comparisons  between  observation  and  theory  are  as  close  as 
could  be  expected  when  we  consider  the  evident  impurities  of  the 
several  minerals.  Lavenite  can  be  formulated  in  a  similar  manner, 
but  the  result  is  doubtful.  The  mineral  contains  impurities  which 
can  not  be  easily  disposed  of  by  mere  calculation.  New  analyses  are 
needed  on  purer  material. 

It  is  worth  noting,  before  going  further,  that  these  new  formulae  for 
the  "zircon  pyroxenes"  are  curiously  similar  to  those  developed  for 
garnet,  epidote,  meionite,  spodumene,  and  similar  minerals.  In  these 
zircosilicates  the  trivalent  radicles^Zr — F  and  ^Cb=0  play  the  same 
part  that  aluminum  and  ferric  iron  play  elsewhere.  The  analogy  is 
suggestive  but  may  be  nothing  more. 

The  typical  sihcate  of  thorium,  thorite,  or  orangite  is  an  unsatis- 
factory species  on  account  of  its  wide  variations  in  composition.  It  is 
commonly  supposed  that  the  mineral,  as  it  exists  in  nature,  has  been 
derived  from  an  original  ThSi04  by  hydration,  and  that  ThSi04  was 
isomorphous  with  zircon.  The  nearest  approach  to  the  type  is  found 
in  orangite,  which  may  perhaps  be  regarded  as  a  thorium  cyrtolite 
or  thorium  malacone. 

Yttriahte  is  another  thoriferous  mineral,  which,  however,  seems  to 
be  a  mixture  of  two  salts.  Its  emphical  formula  is  that  of  an  ortho- 
trisihcate,  and  as  given  by  Hillebrand's  ^  analysis,  it  is  not  far  from 

ThSi207R  +  4Y2SiA; 


U.  S.  Geol.  Survey  BuU.  262,  p.  61, 1905. 


120  THE   CONSTITUTION   OF   THE   NATURAL   SILICATES. 

K,  being  Fe,  Mn,  Pb,  and  Ca.  The  second  of  the  two  compounds  is 
analogous  to  thortveitite,  Sc2Si207.  The  actual  jnineral  contains 
many  impurities,  so  that  its  assumed  constitution  needs  to  be  verified 
by  analyses  of  better  material,  if  that  should  ever  be  found.  Steen- 
strupine  is  stUl  another  silicate  containing  variable  quantities  of 
thorium,  but  the  analyses  are  discordant  and  unreducible  to  any 
simple  formula. 

Mackintoshite,  a  silicate  of  thorium  and  uranium,  may  perhaps  be 
represented  by  the  formula  UO2.2ThO2.3SiO2.3H2O,  and  thorogum- 
mite  appears  to  be  a  hydration  derivative  of  it.  Both  minerals  are 
closely  related  to  thorite,  which  sometimes  contains  noteworthy  quan- 
tities of  uranium.  The  corresponding  salts  of  quadrivalent  uranium 
and  thorium  are  probably  isomorphous.  Uranium  functions  as  a 
hexad  element  in  uranophane,  CaO.2UO3.2SiO2.6H2O,  and  the  mineral 
is  possibly  a  basic  orthodisilicate,  containing  the  dyad  radicle  UO2. 
Its  formula  then  becomes  Ca(U02)2Si207.6H20.  Nsegite  is  a  complex 
zircosilicate,  containing  uranium,  thorium,  yttrium,  and  columbium, 
to  which  no  definite  formula  can  as  yet  be  assigned. 

To  the  titanium  silicates  astrophyllite,  Johns trupite,  and  rinkite 
formulae,  Uke  those  given  to  zircon  and  its  derivatives,  are  assignable. 
Indeed,  this  has  already  been  done  for  astrophyUite  by  Brogger,  who 
writes  the  formula  Ti(Si04)4R' '411^4.  This  seems  to  be  the  dominant 
molecule  in  astrophyUite,  which,  however,  varies  in  composition.  To 
the  Colorado  mineral  we  may  more  precisely  give  the  formula 

Si04=FeH  OH 

/si04^FeK  .                /si04=FeH 

^  ''''^Si04=FeNa  +              ^  '^'^Si04^FeH 

Si04=FeH  Si04^FeH 

which  requires  the  following  percentage  composition: 

SiOa 34. 30 

TiOa 12.20 

FeO 4L 16 

K2O : 5.36 

NaaO 3.  55 

H2O 3. 43 

100. 00 

Some  iron  is  replaced  by  manganese,  and  ferric  iron,  perhaps  as 
=Fe — OH,  is  also  present.  In  the  fluoriferous  astrophyUites  the 
fluorine  should  replace  hydroxy!. 


SILICATES  OF   TETRAD  BASES.  121 

Johnstnipite  and  rinkite  are  both  fluoriferous,  and  both  contain 
earths  of  the  cerium  groups.  In  johnstnipite,  a  little  ZrOj,  ThOj,  and 
CeOa  replace  some  TiOa-     For  Johns trupite  the  expression 

/siO,=CaJ^a 
^^SiO.^CaNa 
Si04=CaH 

agrees  well  with  the  ratios  given  by  analysis.  .  In  rinkite  we  have, 
with  great  probability,  the  mixture 

Si04=CaNa  SiO,=Ti— F 

/si04=CaNa  ^ /siO,=Ca(CeF2) 

v^Si04=CaNa  v  SiO^^CaCCeF^) 

Si04=CaNa  Si04=Ca(CeF2) 

Even  the  complex  mosandrite  reduces  to  the  same  general  type, 
agreeing  very  closely  with 

OH  F 

/siO^^HNaCCeO^H,)  .              /siO^^CaH 

^Si04=HNa(Ce02H2)  "^      ^  ^Si04=CaH 

Si04^HNa(Ce02H2)  Si04=CaH 

in  which  R  =Ce^^  :  Zr  :  Ti  :  :  1  :  2  :  2.  For  each  of  these  species  the 
pubhshed  analyses  agree  well  with  the  composition  calculated  from 
these  formulae. 

Amo^g  the  titanosiUcates,  neptunite  appears  to  be  a  meta-com- 
pound,  analogous  in  general  structure  to  astrophyllite.  Its  formula, 
which  is  sharply  in  accord  with  the  analysis,  is 

SiOg— R' 
/siOg 

'r^<sio;>^" 

SiOa— R' 

where  R'  =  Na,  K,  and  R"  =  Fe,  Mn. 

There  is  still  another  group  of  titaniferous  silicates  which  seems  to 
be  unconformable  with  the  foregoing  scheme  of  interpretation. 
Titanite,  the  typical  member  of  the  group,  has  the  empirical  formida 
CaTiSiOg,  for  which  two  distinct  structures  have  been  proposed. 
One  regards  the  mineral  as  the  calcium  salt  of  an  acid,  HgTiSiOg, 
analogous  to  HaSigOg,  and  the  other  treats  it  as  a  basic  orthosilicate, 

Ca==Si04=TiO, 


122 


THE   CONSTITUTION    OF   THE   NATURAL   SILICATES, 


A  careful  study  of  the  recorded  analyses  of  titanite  leads  me  to  prefer 
the  orthosilicate  expression,  for  the  actual  ratios  vary  in  a  way  which 
indicates  a  replacement,  sometimes  of  Ca  and  sometimes  of  TiO  by 
other  bases.  According  to  the  other  formula,  only  the  calcium  should 
be  replaceable.  This  variability  of  ratio  is  well  shown  by  some  of 
the  varieties  of  titanite,  such  as  grothite,  alshedite,  and  eucolite- 
titanite,  but  the  data  are  not  absolutely  conclusive.  If,  however, 
titanite  is  a  basic  orthosilicate,  it  should  be  classed  with  the  orthosalts 
of  dyad  bases.  On  the  other  hand,  the  acid  character  of  the  titanium 
is  suggested  by  the  remarkable  hydration  derivative  of  titanite, 
xanthitane,  of  which  the  composition  is  approximately  represented 
by  the  formula  Al^TiA^Hg. 

For  tscheffkinite,  as  shown  by  Eakins's  analyses,  the  composition  is 
approximately  (FeCa)3Ce6Ti4Si6032,  but  the  constitution  of  the 
mineral  is  very  doubtful.  Keilhauite  appears  to  be  like  titanite, 
with  Ca  or  TiO  replaced  by  R'"OH  or  K^'OgHj,  but  the  analyses  are 
widely  discordant. 

Benitoite,  having  the  simple  and  definite  formula  BaTiSigOg,  may 
be  either  a  metasilicate  or  else  a  trisilicate, 

Ba  =  Si308  =  TiO. 

The  trisilicate  formula  is  analogous  to  that  of  titanite  and  is  there- 
fore to  be  preferred. 

Nasarsukite  is  a  mineral  of  rather  imcertain  relations,  but  appar- 
ently a  trisilicate 


< 


i308  =  N"a2 
i,0«  =  Na2 


x^b^3w  8  —  ^.11*2 


in  which  about  one-sixth  of  the  sodium  is  replaced  by  the  univalent 
group  — Fe=0.  The  ideal  sodium  salt  compares  with  the  analysis 
of  the  actual  mineral  by  Christensen  as  follows: 


Found. 

Calculated. 

SiOo    

6L63 

14.00 

6.30 

.28 

.47 

.24 

16.12 

.29 

.71 

63.83 

TiOo 

14.19 

FeoOo 

AI2O3 

MnO                                               .                           

2L98 

MgO                      .                

NaoO.            

H^O 

f!  .:;:....:.. 

Less  0    

100.04 
.30 

100.  00 

99.74 

SILICATES  OF   TETRAD  BASES.  123 

Leucosphenite,  which  is  related  morphologically  to  eudidymite,  is 
probably  also  a  trisilicate,  although  its  formula  has  been  written 

Na,Ba(TiO)2(SiA)5. 

This  formula  is  difficult  to  interpret  structurally,  and  a  more  rational 
expression,  which  also  fits  the  analysis  fairly  well,  is  as  follows: 

Na3Ba^Ti,(Si303),. 

Lorenzenite,  an  orthotrisihcate,  appears  to  be  a  crystalline  mixture 
of  two  salts,  thus : 

4  Na2(TiO)2Si207  +  Na2(ZrO)2Si207. 

Several  other  minerals  of  more  obscure  character  remain  to  be 
mentioned  here.  Molengraafite  ^  is  essentially  a  titanosilicate  of 
sodium  and  calcium,  which  may  perhaps  have  the  empirical  formula 
HNa3Ca4Ti3Si402o,  if  replacements  and  obvious  impurities  are  left 
out  of  account.  Its  true  character  is  quite  uncertain.  Epistolite  is 
a  complex  columbosihcate,  which  is  not  far  from  Hi3Na6Cb3Si5027. 
Chalcolamprite  and  endiolite  are  also  columbosiHcates  of  doubtful 
constitution.  Possibly  the  phosphato-sihcates  erikite  and  britholite, 
and  perhaps  also  steenstrupine,  should  be  classed  with  them. 

» See  Brouwer,  Centralb.  Mineralogie,  1911,  p.  129. 


APPENDIX. 

A  number  of  well-defined  silicates,  of  uncertain  constitution,  are 
difficult  to  place  in  any  of  the  classes  covered  by  preceding  chapters. 
They  naay  be  briefly  sunamed  up  as  follows: 

Thaumasite. — Empirical  formula:  CaSi03.CaS04.CaC03.15H20. 

This  can  be  written  structurally,  regarding  the  water  as  extrinsic, 
as  in  some  respects  analogous  to  woUastonite,  but  such  a  formula 
would  lack  the  evidence  necessary  to  sustain  it. 

Tseniolite. — ^A  silicate  of  magnesia  and  the  alkalies  of  uncertain 
composition.     The  one  analysis  of  it  is  incomplete. 

Bakerite. — ^A  borosilicate  of  calcium.  The  empirical  formula  is 
8CaO.5BA.6SiO2.6H2O. 

Roeblingite .' — This  species  is  unique  in  that  it  contains  a  sulphite 
radicle.  It  is  regarded  by  Penfield  and  Foote  as  a  mixed  siUcate  and 
basic  sulphite,  5  H2CaSi04  +  2  CaPbOCSOg). 

This  can  be  given  a  structural  formula,  which,  however,  would  have 
little  real  significance. 

Langhanite.' — A  silicate  of  manganese  and  iron  containing  antimony. 
AlHed  on  crystallographic  grounds  to  hematite  and  ilmenite. 

A  multitude  of  other  silicates  have  been  described  as  species,  but 
without,  as  yet,  securing  fall  recognition.  Some  of  these  are  doubt- 
less mixtures  or  impure  varieties  of  well-known  minerals,  but  others 
may  be  ultimately  established  as  good  and  definite  compounds.  A 
discussion  of  the  analyses,  without,  experimental  investigation  of  the 
various  minerals,  would  have  very  uncertain  value.  I  therefore 
omit  these  doubtful  species  from  consideration. 
124 


INDEX  TO  SPECIES. 


A.  Page. 

Acmite 97, 98, 103 

Actinolite 100, 106 

iEnigmatite 103 

Agnolite 106 

Agricolite 86 

Akermanite 34 

Alamosite 106 

Albite 36, 37, 40, 42, 43, 48, 98 

Alexandrolite 84 

Alipite 106 

AUanite 25 

AUophane 84 

Almandite 25 

Alshedite 122 

Alurgite 54 

Alvite 114 

Amesite 65 

Amphibole 99, 100, 101, 102, 103, 104, 105 

Analcite 37, 38, 39, 40, 41, 42, 43, 46, 47, 48, 98, 108 

Anaphorite 104 

Andalusite 15, 19, 22, 24, 31, 51, 72, 74, 75, 77, 84 

Anderbergite  114 

Andradite 25 

Annite 55 

Anorthite 26,30,32,36,37,41,46 

Anthophyllite 100, 105 

Anthosiderite 84 

Antigorite 92 

Aphrodite 94 

Aphrosiderite 64 

Apophyllite 108 

Arctolite 34 

Ardennite 78, 79 

Arfvedsonite 103 

Arizonite 86 

Aspidolite 53 

Astochite 100,101 

Astrolite 81 

Astrophyllite....- 120 

Auerbachite 113, 115 

Augite 26, 96, 99 

Axinite 65, 72 

B. 

Babingtonite 98 

Bakerite 124 

Barkevikite 103 

Barsowite 74 

B  arylite 80, 81 

Barysilite 107 

Batavite 77 

Bavenite. 49 

Beckelite 85 

Bementite 92 

Benitoite 122 


Page. 

Bertrandite 89, 110 

Beryl 81 

Biotite 24,51,52,53,54,58,66 

Bityite -. 78 

Brewsterite 46 

Britholite 85 

Bronzite 96 

Brunsvigite 60,61 

C. 

Calamine 89, 90 

Cancrinite 27,28,34 

Cappelinite 70, 71 

Carnegieite 21, 23, 45, 48 

Caswellite 59 

Catapleiite 71, 115, 116 

Celadonite 86 

Celsian 35, 46 

Cenosite 85 

Cerite 84 

Chabazite 41, 47, 48, 108 

Chalcolamprite 123 

Chamosite 65 

Chloritoid 56, 57 

Chloropal ' 84 

Chlorophaeite 65 

Chlorophyllite 79,80 

Chondrodite 88, 93 

Chrysocolla 106 

Chrysotile 92 

Cimolite 84 

Clinocblore 59, 65 

Clinohedrite 90 

Clinohumite... 88 

Clintonite 56 

Coimarite HI 

Cookeite 67,78 

Corundopbilite 62, 65 

Cossyrite - 103 

Crocidolite 102 

Cronstedtite 60 

Crossite 102 

Cryoi^yllite 55 

Cummingtonite 109 

Cuspidine 109, 110 

Custerite 107 

Cyrtolite 113,114 

D. 

Danalite 89 

Danburite 74 

Dannemorite 100 

Daphnite ^ 

Datolite 90 

Delessite 60' 61 

Deweylite H^ 

125 


126 


INDEX   TO   SPECIES. 


Page. 

Diabantite. 62, 63 

Didymolite 78 

Diopside : 96,99, 100 

Dioptase 91 

Dudleyite 57 

Dumortierite 65, 73 

E. 

Edingtonite 42, 43, 46 

Ekmanite 63,64 

Elaeolite 22, 27, 28, 37 

Elpidite 115, 116 

Endeiolite 123 

Enstatite 94, 96, 99 

Epichlorite 64 

Epididymite 112 

Epidote 24, 25, 29, 31, 36 

Epiphanite 65 

Epistilbite 46 

Epistolite 123 

Erikite 85,123 

Erionite 46 

Euclase 90 

Eucolite : 116 

Eucolite-titanite 122 

Eucryptite 19,21, 51, 98 

Eudialyte 71, 115, 116 

Eudidymite 112 

Eulytite 86 

Euphyllite 52 

Euralite ^. 65 

F. 

Fassaite 33, 105 

Faujasite 42, 43 

Fayalite 87 

Foresite 49,50 

Forsterite 87, 88 

Fowlerite 96 

Friedelite 91 ,  92 

Fuchsite 51 

G. 

Gadolinite 90, 91 

Gageite 88 

Ganomalite 107, 108 

Ganophyllite 49,50,53 

Garnet 24,25,26,29,30,32,34,56,65 

Gamierite 94 

Gastaldite 103 

Gehlenite 31, 32, 33, 34 

Genthite Ill 

Gismondite 45, 48 

Glaucochroite 87 

Glauconite 86 

Glaucophane 101, 102 

Gmelinite 41, 47 

Gonnardite 50 

Grandidierite 76, 77 

Griqualandite 102 

Grochauite 61 

Grossularitc 25, 33 

Grothite 122 

Griinerite 100 

Guarinite " 116, 117 

Gyrolite 108 

H. 

Hallite 59 

HaUoysite 83 


Page. 

Hancockite 25 

Hardystonite 107 

Harmotome 41, 43, 44, 46 

Harstigite 75 

Hastingsite 101, 103 

Haughtonite 53 

Haiiynite 26 

Hedenbergite 96,99 

Hellandite 85 

Helvite 89 

Heulandite 41, 46, 48 

Hibscbite 45, 76 

Hillebrandite 89, 106, 107 

Hiortdahlite 116, 117 

Hisingerite 84 

Homllite 90, 91 

HuUite 65 

Humite 88 

Hyalophane 35, 46 

Hyalotekite 110,111 

Hydrobiotite 58 

Hydrocastorite 82 

Hydronephel  ite 26, 40, 42, 43 

Hydrophlogopite 58 

Hypersthene 96 

T. 

Tgelstromite 87 

Ilvaite 89,90 

Inesite 109 

lolite 79 

Irvingite 55 

J. 

Jadeite. 97,98 

Jefferisite 58 

JeffersoBite 96 

Jobnstrapite 120, 121 

K. 

Ksersutite 104 

'Kainosite 85 

Kaliopbilite 19, 21, 22 

Kaolinite 36, 37, 39, 40, 57, 58, 64, 83, 84 

Karyocerite 70, 71 

Karyopilite 91 

Keilbauite 122 

Kentrolite 85 

Kerolite 94 

Kerrite 58 

Klementite 65 

Knebelite 87 

Komerupine 76, 77,96 

Kotscbubeite 60 

Kryptotile 19, 22, 52, 76, 83 

Kyanite 15, 23, 24, 31 

L. 

Lagoriolite 24, 25 

L^ngbanite 124 

Lasallite 80 

Laubanite 49 

Laumontite 41, 46, 47, 48 

L&venite 116, 119 

Lawsonite 45, 76 

Lazurite 26 

Lennilite 58 

Lepidolite 54,55 

Lepidomorpbite 52 

Leuchtenbergite 59, 65 


INDEX  TO   SPECIES. 


127 


Leucite 37,38,40,43,98 

Leucophanite 106 

Leucophcenicite gg 

Leucosphenite 123 

Leverrierite  ^ 22  51 

Levynite 47'  4g 

Linosite IO4 

Lorenzenite 123 

Lucasite 5g 

M. 

Mackintosliite 120 

Maconite 5g 

Malacon 113,114,115 

Manandonite 65, 73, 78 

Manganidocrase 31 

Manganophyllite 53 

Margarite 57 

Marialite 36, 37 

Meionite 23, 36, 37 

Melanocerite 70, 71 

Melanolite 60 

Melanotekite 85 

Melilite 26, 31, 33 

Melite 84 

Meliphanite 112 

Mesolite 42, 48 

Metachlorite 61 

Microsommite 27, 37 

Milarite 82 

Minguetite 64 

Molengraafite 123 

Molybdophyllite 89 

Monticellite 87, 99, 105 

Montmorillonite 83 

Mordenite 48 

Mosandrite 121 

Miillerite 84 

Muscovite 19, 

22, 23, 26, 34, 36, 37, 51, 52, 55, 66, 72, 74 

N. 

Naegite 120 

Nasarsukite 122 

Nasonite 107, 108 

Natrolite 26,40,42,43^46,48 

Nephelite. 19, 21, 23, 27, 28, 36, 37, 40, 42, 46, 51 

Nepouite 94 

Neptunite 121 

Newtonite 83 

Nontronite 84 

Noselite 26 

Noumeite 9^ 

p. 

Ofiretite 50 

Okenite 108 

Olivine 26, 94*,  99, 105 

Orangite 119 

Orthoclase 37,48 

Ottrelite 56 

Ouvarovite 25 

P. 

Painterite 59 

Paragonite 19, 22,51, 52 

Partschinite 25,29 

Pattersonite 59 


Page. 
Pectolite 93 

Pennine .....59,60,65 

Petalite •;. g2 

Phenakite §7  gg 

Phengite '52 

Philadelphite sg 

Philipstadite 103 

Phillipsite 41,43,44,45,48 

Phlogopite 51,63,64,59,66 

Pholidolite 107 

Picrosmine 94 

Piedmontite 25 

Pilinite 49 

Pilolite ,. gQ 

PlancMite igg 

Plombierite iQg 

PoUucite 39 

Polylithionite : 55 

Prehnite 24, 29,34,36,38,48 

Prismatine 73  77 

Prochlorite gl,  62^*65 

Prolectite gg 

Protovermiculite sg 

Pseudobrookite gg 

Pseudodeweylite 107,  m 

Pseudoleucite 37 

Pseudonatrolite 59 

Pseudonephelite 22 

Ptilolite 49 

Pycnochlorite 65 

Pyrope 25 

Pyrophyllite gg 

Pyrosclerite 59 

Pyrosmalite 91 

Pyroxene 95,96,97,99,100,104,105 

R. 

Rectorite 22, 83 

Rhodonite 96 

Rhodusite 102 

Rhonite 104 

Richterite 100 

Riebeckite 103 

Rinkite 120,121 

Rivaite 1 10 

Roeblingite 124 

Roscoelite 51 

Roseite 58 

Rosenbuschite 116, 118 

Rubellite 68 

Rumpfite 60 

S. 

Sahlite 96 

Samoite 84 

Saponite 109 

Sapphirine 73 

Sarcolite 24 

Scapolite 31, 33, 34, 36, 51 

Schefferite... 96,110 

Schizolite , 96 

Schorlomite 26 

Schrotterite 84 

Scolecite , , 42,43,48 

Serendibite 65,73,78 

Sericite 51 

Serpentine 65,92,93,94 


128 


INDEX  TO  SPECIES. 


Page. 

Seybertite 56 

Sheridanite 64 

Siderophyllite 53 

SiUimanite 15, 23, 24, 31, 83 

Sodalite 26, 27, 28, 29, 37, 40, 42 

Soretite 103 

119 

25,29 

Sphenoclase 81 

Spinel 56,65 

Spodiophyllite 107 

Spodumene 97,98,99 

Spurrite 89 

Staurolite 74, 75 

Steenstrapine 123 

Stellerite 50 

Stilbite 41, 43, 44, 45, 48 

Stilpnomelane 63,64 

Stokesite 115 

Strigovite 64 

Syntagmatite 101 

T. 

Taeniolite 124 

Talc... 105,106 

Talc-chlorlte 63 

Taramellite 107 

Tawmawite 25 

Tephroite 87 

Termierite .' 84 

TMlenite 85 

Thaumasite : 124 

Thomsonite 45 

Thorite 119 

Thorogummite 120 

Thortveitite 85 

Thuringite 62, 63 

Titanite 121, 123 


Page. 

Topaz 19,22, 23,51 

Tourmaline 65,66,67,68,69,78 

Tremolite 100, 101, 105 

Trimerite 87,89 

Tritomite 70, 71 

Troostite 89 

Tschefifkinite 122 

U. 
Uranophane 120 

V. 

Vaalite 59 

Vermiculite 58, 59 

Vesuvianite i30, 31, 34, 65 

Villarsite 92 

W. 

Waldheimite 101 

Weinbergerite 107 

Wellsite 43,44 

Westanite 83 

Willcoxite 57 

Willemite 87, 89 

Woerthite 83 

Wohlerite 116, 118, 119 

Wolchonskoite 86 

WoUastonite 95, 96 

X. 
Xanthophyllite 56,65 

Y. 
Yttrialite 119 

Z. 

Zeophyllite Ill,  112 

Zinnwaldite 54, 55 

Zircon 113, 114, 115 

Zoisite 25,29 

Zungite 24, 29 


O' 


5{)4"to 


